Mary is a super genius and the older twin sister of Susan and also the older sister of Johnny. Mary has been shown to be more empathetic and sympathetic than her sister. Like her sister, Mary harbors a deep love bordering on obsession for the Tests' next-door neighbor Gil Nexdor. Like her sister, she attends school at the Porkbelly Institute of Technology (a.k.a "Porkbelly Tech" for short). Mary is prone to believing in the less scientific and more science-fictional, which, although it receives scorn from her twin, often proves to be correct in the end. Mary seems to be more level-headed than her twin Susan, and Mary tends to be more conservative, as she speaks her mind much less than Susan. Though Mary wears pants, she seems to be more "girly" than her twin Susan. This is revealed when she decides to make a normal dress in the episode Princess Johnny. Mary is the older twin of the two. Her age appearance appears to be somewhere around fifteen years old. Susan and Mary test are very smart. They are smarter than Bling Bling Boy and Mr. Black and Mr. White, but one thing for sure: they don’t know how to get Gil's love. Susan and Mary Test would both like to be Gil's girlfriend, which often gets them into tied fights, proving her brother's intelligence of knowing that she and her sister can't both have Gil as a boyfriend.
Mary has long curly red hair and teal eyes. She wears a light yellow shirt with a blue moon on it, baggy blue jeans, light olive green sneakers with the shape of an ivory-colored crescent moon, and a white lab coat. She wears a moon-shaped barrette in her hair. She and Susan both wear glasses. When she is seen without her glasses in Downhill Johnny, her eyes appear as green dots. But in Johnny's Got a Wart!, her eyes are like everyone else's. Her eye color is the same as her mother, Lila's. Her bathing suit is a pink two-piece bikini. Her disco outfit is a purple tank top that reveals her midriff and navel, with magenta shorts, and silver boots. She also has outfits suited for certain controlled experiments, such as a high-tech diving suit, and a skin-tight yellow hazmat suit. Both suits have her trademark moon on the side. When handling bio hazardous materials, usually while performing surgeries, she dons a surgical mask and a pair of cyan-blue latex gloves.
In Season 1, Mary was portrayed by Brittney Wilson. During Seasons 2-4, the role was taken over by Ashleigh Ball. Wilson reprised the role in Season 5, with Ball taking it back for Season 6.
Mary and Susan have a wonderful sisterly relationship. As twins, Mary can often read Susan's mind (like "twin telepathy"), and it's typical that they're both geniuses. Working in a lab all day long can make Mary and Susan totally exhausted, and so they don't have the energy to help Johnny with his problems. Also, Mary and Susan do almost everything together, like staring at Gil (their biggest crush), or conducting new experiments in their lab. However, despite their relationship, they can sometimes bicker when it comes to their differences.
Mary has a complicated relationship with her younger brother Johnny, although she somehow seems most compassionate to Johnny more than anyone else (unless Susan makes a accurate point). Also, Johnny tends to be nicer to Mary than Susan for that cause. Also, Johnny doesn't annoy Mary that much. She loves her younger brother very much.
They do seem to get along well and tend to come up with plans on getting Johnny out of trouble.
Mary loves her father, but despises his meatloaf and his constant punishments for conducting experiments.
She and her mother has a healthy mother-daughter relationship. Unlike Hugh, she doesn't go as far as to punish her daughters but only when necessarily.
Like Susan, she is head over heels over Gil. She does things with Susan from watching him from the satellite in their lab and trying to hang out with him. Also, despite the fact that Gil has been living next door to the girls for most of his life, he often forgets them, much to their dismay.
She despises him since whenever he comes to woo Susan, he cuts into their time inventing.
- Telekinesis ("Johnny X Strikes Back")
- Along with Susan, her catchphrase is "We're such geniuses".
- She and Susan are extremely ticklish, as was revealed in "Johnny vs. The Tickler".
- She and Susan are revealed to suffer from plantar hyperhidrosis, "stinky feet", in "Johnny Unplugged".
- Her identical twin sister is Susan, but she's still the oldest child in the Test family.
- She and Susan are two of the seventeen characters in the series to be in pairs. The other two are Johnny and Dukey, Joni and Dutchy, Sissy and Missy, Hugh and Lila, Black and White, Dark Vegan and toast (possibly), Dawg and Bone, and Mittens and Albert. (Seventeen because toast doesn't count as a character).
- Despite treating Johnny as a guinea pig, she seems to be nicer to him than Susan is in many occasions.
- In "Johnny vs. Dukey", she sides with Johnny in an insane game of 'Say Uncle' against Susan and Dukey.
- She's more quiet and level-headed than Susan.
- She has switched bodies only once so far in the series (she switched bodies with Johnny).
- She might like Gil a little less than Susan, due to her personality.
- She hardly gets mad at Johnny, unlike Susan. But she only gets enraged at him when he often ruins her and her sisters' conducted experiments.
- She believes more in science-fiction rather than just plain science.
- In many cases, she's more creative than Susan when it comes to inventing gadgets.
- In "Johnny's Big Dumb Sisters", Mary, along with her sister, appear to be way dumber than their brother, Johnny, due to the accident caused by Johnny, himself.
- Her steel blue moon-shaped barrette ties up her extremely curly red hair, like a bushy ponytail. This was shown in Johnny Escape From Bling Bling Island.
- She and Johnny both wear baggy jeans, except Mary's are steel blue, while Johnny's are evergreen.
- Mary is generally the most well liked character in the fandom due to her being more level-headed then any of the other characters.
- So far in the series, Mary hasn't had any episodes primarily dedicated to her, unlike most of the other characters on the show.
- She has had the most voice-actor changes of any character:
- Ashleigh Ball took over the role from Britteny Wilson in Season 2, but left in Season 5 to fulfill her commitments to My Little Pony: Friendship is Magic. Ball returned in Season 6 during an extended break in production for MLP.
- In Johnny's Treasure, it is revealed that Mary can play the flute while Johnny and Dukey revealed she was a "Band Dork".
- In the episode "Johnny Test: Party Monster" Mary mentioned that the test children have an aunt named Bernice. It is unknown if this aunt is related to them on their mother or father's side of the family.