Susan and Mary Test: Johnny Test’s Genius Twin Sisters

Let’s talk about Susan and Mary Test, the genius twin scientist sisters who quietly carry half of the Johnny Test franchise on their backs.

I’ll be honest. Johnny Test gets remembered for the chaos. The dog who talks. The neon hair color. The endless reuse of “whip-cracking” sound effects. But when I actually rewatch the show, I keep noticing that the engine driving every plot is the same. Susan and Mary Test. The twin sisters whose inventions kick off almost every adventure in the series.

In this post, I’m breaking down who Susan and Mary Test are, their personalities, their ages, what makes them different from each other, and why they matter so much to the show.

Who Are Susan and Mary Test?

Susan and Mary Test are the older twin sisters of Johnny Test in the American-Canadian animated series Johnny Test, which originally debuted in 2005 and ran through multiple seasons (with a Netflix revival in 2021).

They’re brilliant. They’re inseparable. They have a fully equipped home laboratory in the Test family basement where they build everything from jetpacks to time machines to mind-control devices. And almost every Johnny Test episode kicks off because one of their inventions gets borrowed (read: stolen) by Johnny and his talking dog Dukey.

The Distinct Personalities of Susan and Mary Test

Even though Susan and Mary are identical twins, the writers gave them very different personalities. That contrast is honestly what makes them work as characters.

Susan Test (the bold one)

Susan is the more assertive of the two. She’s the type to dive headfirst into an experiment, push for bigger risks, and take the lead when something needs done. She’s a bit of a perfectionist, which sometimes causes friction with her chill twin.

Her personal style leans tomboy. She likes rock music, skateboarding, and video games. She also has a slightly louder voice and a sharper temper, especially when Bling-Bling Boy (her unwanted admirer Eugene Hamilton) shows up to bother her.

Mary Test (the level-headed one)

Mary is the calmer, more empathetic twin. She thinks about consequences. She considers how an invention will affect people. She’s usually the voice of reason when Susan wants to launch some half-tested device at Gil’s house just to get his attention.

Mary leans a little more traditionally girly than Susan, and she communicates with Johnny in a gentler way. But don’t mistake that for weakness. She’s just as brilliant as Susan and can build anything Susan can.

Together, they balance each other in a way that makes their partnership feel earned, not just written. Susan brings the drive. Mary brings the wisdom. Both bring the science.

The Twin Sisters Who Power Every Johnny Test Adventure

Here’s the thing about Johnny Test that not enough people acknowledge. Susan and Mary are technically the secondary characters of the show, but they’re also the reason any of it happens.

Every episode plot is basically:

Their home laboratory has produced jetpacks. Time machines. Cloning devices. Mind-control helmets. Superpowers in pill form. Shrink rays. Genetic modification serums. Almost anything you can imagine. The lab is essentially a Marvel-tier R&D facility hidden in a Canadian suburban basement.

The twins’ creativity is the engine. Johnny is just the chaos agent who sets it off.

Why Susan and Mary Test Matter for STEM Representation

One thing I want to give the show real credit for: Susan and Mary Test were genuine STEM-representation characters before that was a thing people talked about much in kids’ animation.

The twins are positioned as genius scientists first, sisters second. They build things. They run experiments. They’re explicitly the smartest characters in the show, including the adults. And they’re never positioned as “weird” for being into science.

The show doesn’t make a big speech about it. It just shows them being brilliant, episode after episode. That’s actually the more effective approach. Empowering girls in cartoons doesn’t always require a monologue. Sometimes it’s just showing them as competent without making it weird.

The Twin Bond and How It Shapes the Show

The Susan-and-Mary dynamic is one of the most consistently well-written sibling relationships in early-2000s cartoons. They support each other, finish each other’s sentences, and protect each other (especially from Bling-Bling Boy). But they also bicker, get jealous, and disagree like real siblings.

That balance matters. A lot of cartoons turn twins into a single “two-headed” character with shared dialogue. Susan and Mary aren’t that. They’re two distinct people who happen to look identical and share a lab.

Their bond also shapes how they treat Johnny. They tease him relentlessly, but when things actually go wrong, they’re the first ones to help him fix the mess. That sibling loyalty is the emotional anchor of the whole show.

Why Susan Test Stands Out

1. She’s fearless and tough, and she’s not afraid to call people out when they’re wrong.
2. She has a distinct tomboy energy that gave the show one of its more memorable visual designs.
3. She enjoys rock music, fighting, skateboarding, and video games, which actually overlaps with Johnny’s interests in a way that gives their sibling dynamic real depth.
4. She handles harassment from Bling-Bling Boy with comedic exasperation that sells the joke every time.
5. She has strong reasoning skills and tends to be the “logically correct” twin even when her emotional choices are off.
6. She’s protective of Johnny in her own gruff way, especially when his use of her inventions puts him in real danger.
7. She holds her own against the more wholesome Mary without ever feeling like the “mean” sister.
8. She has a clear character arc across the series, especially around her crush on Gil.

Why Mary Test Stands Out

1. She’s more level-headed and empathetic than Susan, which gives the twin pair real balance.
2. She communicates with Johnny more kindly, which makes her the “approachable” sister.
3. Brittney Wilson’s voice work gives Mary a distinct softness without making her sound passive.
4. She tends toward more traditionally feminine hobbies, providing a deliberate contrast to Susan.
5. She shows visible care and compassion for Johnny, often defending him when Susan loses patience.
6. She works alongside Susan on every major invention, proving she’s just as scientifically gifted.
7. She has a strong loyalty streak, especially when Bling-Bling Boy flirts with Susan.
8. She gives the show its emotional grounding without ever being preachy about it.

The Test Siblings’ Impact on Johnny’s Life

Johnny Test would be a completely different show without Susan and Mary. The series leans on the twins for:

The Test family dynamic, especially the Susan-Mary-Johnny trio, is the actual emotional core of the show. The neon hair and the talking dog get the marketing attention, but the sibling chemistry is why the show kept getting renewed.

Frequently Asked Questions

How old are Susan and Mary Test?

Susan and Mary Test are 13 years old in seasons 1 through 6 of Johnny Test, and they age up to 14 starting in season 7. They’re four years older than their younger brother Johnny.

Are Susan and Mary Test identical twins?

Yes. Susan and Mary Test are identical twin sisters. The show distinguishes them through their hair (Susan has straighter hair, Mary has more curled bangs), their clothing style, and their personalities. Susan is the assertive tomboy. Mary is the more traditionally feminine and empathetic twin.

Who voices Susan and Mary Test?

Maryke Hendrikse voices Susan Test, and Brittney Wilson voices Mary Test in the original Johnny Test animated series. Both voice actresses returned for the Netflix revival series in 2021.

What kinds of inventions do Susan and Mary Test build?

The Test sisters have built dozens of fantastical inventions across the series, including jetpacks, time machines, cloning devices, mind-control helmets, shrink rays, superpower-granting serums, genetic modification machines, and more. Most episodes kick off because Johnny and Dukey “borrow” one of their inventions without permission.

Why do Susan and Mary Test both have a crush on Gil?

Gil Nexdor is the Tests’ next-door neighbor and the unwitting object of both twins’ affection. The shared crush is a recurring source of competition between Susan and Mary, and many of their inventions are secretly built to impress Gil (who is famously oblivious to both of them).

Who created Susan and Mary Test?

The Test family characters were created by Scott Fellows, who developed Johnny Test for Warner Bros. Animation. The series originally aired on Kids’ WB and The CW4Kids before moving to Cartoon Network, and was revived by Netflix in 2021.

Are Susan and Mary Test good role models?

For kids interested in STEM, absolutely. The Test sisters are explicitly positioned as the smartest people in the show, regularly outperform adults in scientific and engineering work, and are never mocked or undermined for their intelligence. They’re some of the better young-girl scientist representations in early-2000s American animation.