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 Test's 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, which often proves her brother's intelligence of knowing that she and her sister can't both have Gil as a boyfriend.
Mary has a light yellow t-shirt with a blue moon on it, long curly red hair, and a moon-shaped barrette. 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. She often wears baggy blue jeans and light olive green sneakers with the shape of an ivory-colored crescent moon and a white lab coat. Her bathing suit is a pink two piece bikini , and 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 protrayed by Brittney Wilson. During Seasons 2-4, the role was taken over by Ashleigh Ball. Wilson returned to the role in Season 5. In Season 6, Ball later returned to voice Mary again.
- Along with Susan, her catchphrase is "We're such geniuses".
- She and Susan are extremely ticklish, as was revealed in "Johnny vs. The Tickler".
- Her identical twin sister is Susan, but she's still the oldest child in the Test family.
- She and Susan are two of the fourteen characters in the series to be pairs. The other two are Johnny and Dukey, Joanie and Dutchie, Sissy and Missy, Hugh and Lila, Black and White, and Mittens and Albert.
- 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 switches bodies only once so far in the series (the first being 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".