Susan and Mary Test are the two people most responsible for making Johnny Test’s life exciting, dangerous and completely unpredictable. They are his genius twin sisters, and the inventions coming out of their home laboratory can solve almost any problem. They can also create a much larger one before breakfast.
That is what I like about them. Susan and Mary are not simply there to hand Johnny a gadget whenever the story needs one.
They have their own goals, different personalities and a habit of using their younger brother as a test subject when he wants something from them.
Sometimes Johnny sneaks into the lab and causes the disaster himself. Other times the twins make a scientific breakthrough that works exactly as intended until someone takes it too far. Either way, the three Test siblings usually end up cleaning up the same mess together.
Who Are Susan and Mary Test?
Susan and Mary are Johnny’s older twin sisters and the daughters of Hugh and Lila Test. They are teenage scientists who work from an advanced laboratory built inside the family home.
The twins can create almost anything the episode requires, including robots, time machines, cloning devices, growth formulas, mind-control technology and machines that alter Johnny’s body or personality. Their inventions often begin as school projects or attempts to impress their neighbour Gil.
Johnny normally becomes involved because he wants help with a personal problem. He may need to become stronger, taller, smarter or more popular. Susan and Mary agree to help, usually in exchange for him testing something they have not yet proved safe.
- They are Johnny Test’s older twin sisters.
- Both are highly gifted scientists and inventors.
- Their laboratory is located inside the Test family home.
- Johnny and Dukey frequently test their inventions.
- The experiments often create the main problem of an episode.
The original series generally presents Susan and Mary as 13 years old. The franchise uses a loose cartoon timeline, so I would not treat later season numbers as proof that they suddenly age in a consistent way.
How Susan and Mary Are Different
Because the twins work together so often, it is easy to describe them as though they have the same personality. They do not.
Susan is usually the more forceful and competitive of the pair. She tends to take the lead when presenting an invention, arguing with Johnny or dealing with Bling-Bling Boy. She is impatient when someone ignores her instructions and more likely to turn a disagreement into a challenge.
Mary is normally calmer and more willing to think through the emotional side of a problem. She often questions Susan’s first reaction and can be more sympathetic toward Johnny, although she is not above using him in an experiment.
The difference should not be pushed too far. Susan can be caring, and Mary can become just as excited or reckless when an invention interests her. They are twins and partners before they are opposites.
Susan Test
- More assertive, competitive and quick to take control
- Usually the twin who confronts Johnny directly
- Frequently irritated by Bling-Bling Boy’s unwanted attention
- Often presented as the more tomboyish sister
- Voiced by Maryke Hendrikse in the original series and the Netflix revival
Mary Test
- Generally calmer and more patient than Susan
- More likely to consider how an invention affects other people
- Still willing to experiment on Johnny when science is involved
- Often acts as the softer voice during arguments
- Voiced by Brittney Wilson and later Ashleigh Ball in the original run, with Emily Tennant taking the role in the Netflix revival
I prefer the episodes that allow both sisters to contribute instead of treating Mary as Susan’s assistant. Susan may be louder, but the partnership works best when Mary gets to challenge her or offer the solution she overlooked.
Their Laboratory Drives the Series
The twins’ laboratory is one of the most important locations in Johnny Test. Without it, Johnny would be an ordinary kid with a talking dog. With it, he can become a superhero, travel through time or accidentally threaten Porkbelly before dinner.
Susan and Mary are talented enough to build devices that government scientists and villains want to steal. The joke is that their most advanced technology is often used for something small, such as helping Johnny avoid homework or getting Gil to notice them.
The inventions are not always failures. In many episodes, the machine works perfectly until Johnny ignores the warning label, changes a setting or refuses to return it. The twins can be reckless, but Johnny is frequently the person who turns a controlled experiment into chaos.
- Body-changing formulas and machines
- Robots, clones and artificial intelligence
- Time-travel and teleportation devices
- Superhero equipment used by Johnny X
- Mind-control, growth and intelligence experiments
The show never spends much time asking how two teenagers can build technology this advanced. That is probably the right decision. The lab works because the audience accepts that Susan and Mary can make almost anything, and the fun comes from seeing what Johnny does with it.
Johnny Is Their Brother and Their Favourite Test Subject
The relationship between Johnny and his sisters is more complicated than the original article made it sound. Susan and Mary do care about him, but they are not endlessly patient mentors guiding him through life.
They argue with Johnny, bargain with him and sometimes treat him like convenient laboratory equipment. Johnny responds by sneaking into the lab, borrowing unfinished inventions and ignoring every safety instruction they give him.
That makes the sibling relationship believable underneath all the science-fiction chaos. The three of them annoy one another constantly, but they also know they can rely on each other once a real threat appears.
- Johnny tests inventions in exchange for help or special abilities.
- The twins frequently rescue him after an experiment goes wrong.
- Johnny sometimes saves Susan and Mary from their own creations.
- All three siblings hide laboratory disasters from their parents.
- The arguments rarely weaken their underlying loyalty to one another.
I would not describe Susan and Mary as Johnny’s mentors. They are his sisters. Sometimes they teach him something, sometimes they use him, and sometimes all three are equally responsible for the disaster.
Why the Twins Work So Well Together
Susan and Mary are almost always shown working as a team. They finish each other’s explanations, argue over methods and move around the lab as though they have repeated the same routine hundreds of times.
Their partnership works because their personalities do not completely match. Susan pushes the experiment forward. Mary is more likely to notice the human problem that Susan has ignored. When one becomes distracted by Gil or frustrated with Johnny, the other can keep the project moving.
They also share the same weaknesses. Both can become overconfident, both underestimate Johnny’s ability to misuse a machine and both occasionally build something dangerous for a very silly reason.
The show rarely makes their bond sentimental, which I think suits them. They demonstrate how close they are by continuing to work together after every explosion, mutation and argument.
Susan, Mary and Gil Next Door
A major running joke in the original series is that both twins have a crush on Gil, the handsome boy who lives next door.
Gil is friendly but remarkably unaware of how much attention Susan and Mary give him. The twins may build an invention, change their appearance or design an elaborate plan simply to impress him, only for Gil to misunderstand the situation or walk away before noticing.
This is one area where Susan and Mary become less distinct. Both are intelligent enough to invent world-changing technology, yet neither can communicate normally with the boy next door.
The joke can become repetitive, but it also keeps the twins from feeling too perfect. They may understand advanced physics, but a simple crush can make them as awkward as anyone else.
Bling-Bling Boy Creates a Different Problem
Eugene Hamilton, better known as Bling-Bling Boy, is another young scientific genius. Unlike Gil, he is very aware of Susan and spends much of the series trying to force her to like him.
Susan repeatedly rejects him, and Mary usually supports her sister. His inventions and attempts to impress Susan often become threats that Johnny and the twins must stop.
The original draft described this mainly as Susan “handling harassment,” which is accurate but too mild. Bling-Bling Boy ignores clear rejection over and over again. The show plays much of it for comedy, but Susan has every reason to be angry with him.
- Bling-Bling Boy is a wealthy young inventor and recurring rival.
- He is obsessed with winning Susan’s affection.
- Susan consistently rejects his advances.
- Mary usually defends Susan and dislikes his behaviour.
- His schemes frequently put Johnny or Porkbelly in danger.
Susan and Mary as Girls in Science
It is fair to say Susan and Mary are positive examples of girls in science, but the original article pushed that point so hard that it began to sound like a school presentation.
The twins never need an episode explaining that girls can become scientists. They are simply the smartest people in the room from the beginning. Johnny, Dukey, government agents and villains all depend on their inventions.
That casual confidence is more effective than a speech about breaking stereotypes. Susan and Mary can be brilliant, stubborn, funny, selfish and caring without the show treating intelligence as their only acceptable personality trait.
They also make mistakes. That matters because a character does not become inspiring by being perfect. The twins are allowed to fail, rebuild the machine and try again.
What Do Susan and Mary Look Like?
The sisters have similar faces and red hair, but their clothing and hairstyles make them easy to tell apart.
Susan usually wears her hair shorter and dresses in darker, more practical clothing. Mary wears her hair longer and is normally given a more traditionally feminine outfit.
The visual contrast matches the broad personality difference without making them look unrelated. They still read immediately as twins, especially when standing together in the lab.
The 2021 Netflix revival updated the animation and character designs, but it kept the same basic distinction between them.
Who Voices Susan and Mary Test?
Maryke Hendrikse voices Susan in the original series and returned for the Netflix revival.
Mary had more than one voice actress during the original run. Brittney Wilson performed the character in earlier episodes, while Ashleigh Ball voiced her in later seasons. Emily Tennant took over as Mary in the 2021 revival.
The voice changes are noticeable if you watch the entire series in order, but the character’s calmer delivery remains consistent enough that Mary still feels like the same twin.
Why Susan and Mary Matter to Johnny Test
Johnny Test is named after Johnny, but Susan and Mary create much of the world around him. Their laboratory gives the series its science-fiction ideas, their rivalry with other geniuses introduces villains, and their inventions allow Johnny to become almost anything for eleven minutes.
They also prevent Johnny from becoming unstoppable. He may gain superpowers or an advanced vehicle, but his sisters understand how the technology works and know when he has pushed it too far.
My favourite part of the trio is that nobody stays in one role. Susan and Mary are not always the sensible scientists, and Johnny is not always the reckless test subject. Some episodes reverse those positions, which keeps the family dynamic from becoming completely predictable.
Frequently Asked Questions About Susan and Mary Test
Are Susan and Mary Test twins?
Yes. Susan and Mary are Johnny’s older twin sisters and the daughters of Hugh and Lila Test.
How old are Susan and Mary Test?
They are generally presented as 13 years old in the original series. The show uses a flexible cartoon timeline, so their ages do not advance consistently with the number of seasons.
Which twin is Susan Test?
Susan is usually the more assertive and competitive twin. She has shorter hair and is often presented as more tomboyish than Mary.
Which twin is Mary Test?
Mary is generally calmer and more empathetic. She has longer hair and often acts as the more patient voice during arguments.
Why do Susan and Mary experiment on Johnny?
Johnny often agrees to test their inventions because he wants a gadget, power or favour in return. He also borrows technology without permission, so not every experiment is the twins’ idea.
Who is smarter, Susan or Mary?
The series treats both sisters as super-geniuses. Susan often takes the lead, while Mary sometimes notices practical or emotional details that Susan misses. There is no reliable reason to declare one permanently smarter.
Who do Susan and Mary have a crush on?
Both twins have a long-running crush on their neighbour Gil. Gil is usually unaware of their attempts to impress him.
Does Bling-Bling Boy date Susan?
No. Bling-Bling Boy repeatedly pursues Susan, but she clearly rejects him.
Final Thoughts
Susan and Mary work because they are more than a pair of convenient geniuses. Susan is competitive and direct. Mary is calmer but not passive. Both are brilliant, flawed and capable of creating the problem they later have to solve.
Their relationship with Johnny is equally important. They love their brother, but that does not stop them from arguing with him or attaching experimental machinery to him. Johnny returns the favour by ignoring their warnings and taking inventions he was told not to touch.
That combination gives the series its rhythm. The twins build something impossible, Johnny finds a reckless use for it, and the three siblings work together once Porkbelly is in danger again.
Which Test twin do you prefer, Susan or Mary?
Let me know in the comments.
