|“What can go wrong?”|
|— Mary Test|
Mary is a super-genius twin sister of Susan and the older sister of Johnny. Mary is more empathetic than her sister. Like her sister, Mary harbors affection for the next-door neighbor Gil Nexdor. Like her sister, she attends school at the Porkbelly Institute of Technology (a.k.a Porkbelly Tech). However, Mary is prone to believing more science-fictional, which is often proven. Mary seems to be more level-headed than her twin Susan and less outspoken. She seems to be more girly than her twin Susan. They may be more intelligent but not wise about how to get Gil's love. Susan and Mary Test would both like to be Gil's girlfriend, which often gets them into matched fights.
Mary has long curly red hair tied in a ponytail and green eyes. She wears a pastel yellow shirt with a moon design. blue jeans, olive green sneakers with a moon design, white lab coat and a moon hairclip. 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. Like her mother Lila, Her eye color is green. Her swimsuit is a pink two-piece bikini. Her music video outfit is a purple sports bra, pink shorts, and white platform boots. She also has outfits suited for specific controlled experiments, like a high-tech diving suit, and a latex yellow hazmat suit. Both suits have her trademark moon on the side. When handling biological hazardous materials, usually while doing surgeries, she wears a surgical mask and a pair of cyan-blue latex gloves. As a vampire, Mary wears a long-sleeved black dress that covers her legs, with her dress plus indigo underlines, including a large cape that acts like wings whenever she flies, a small red spider can be adorned on the collar. Her moon also becomes indigo, and her eyes became chartreuse, with two long and sharp fangs sticking out of her mouth.
Mary and Susan have a wonderful sisterly relationship. As twins, Mary can often read Susan's mind (like "twin telepathy"), but it is common that they're both geniuses. Working at the lab all day long can make Mary and Susan fully exhausted, and so they don't need the energy to help Johnny with his issues. Also, Mary and Susan do almost anything together, like staring at Gil (their biggest crush), or supervising new experiments in their lab. However, despite their relationship, for some reasons, they can bicker when it comes to their differences.
Mary has a sophisticated relationship with her younger brother Johnny, although she somehow seems most compassionate to Johnny more than anyone else (unless Susan makes an accurate point). Also, Johnny tends to be nicer to Mary than Susan for that cause. Also, Johnny doesn't annoy Mary that much, although she too can dislike Johnny's antics as well. 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 hates his meat loaf and his constant punishments for supervising experiments.
She and her mother has a nice mother-daughter relationship. Unlike Hugh, she wouldn't go as far as to punish her daughters but only when unquestionably.
Like Susan, she is falling in love with Gil. She does things with Susan from seeing him from the satellite in their lab and trying to hang out with him. Also, despite the fact that Gil had been living next door to the girls for most of his life, he often forgets them, much to their distress.
She is not fond of him since he flirts with Susan and shortens their time inventing.
- Telekinesis ("Johnny X Strikes Back")
- Alongside Susan, her catchphrase is "We're such geniuses".
- She and Susan are very ticklish, as was shown 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 is still the oldest child in the Test family.
- She and Susan are two of the 17 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. (17 because Toast does not 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 backs up with Johnny in an insane game of 'Say Uncle' against Susan and Dukey.
- She is 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 bit less than Susan, due to her personality.
- She is scarcely got mad at Johnny, unlike Susan. But she only gets enraged at him when he often ruins her and her sisters' supervised experiments.
- She believes more in science-fiction rather than a stark science.
- In several cases, she's more creative than Susan when it comes to inventing gadgets.
- In "Johnny's Big Dumb Sisters", Mary, along with Susan, appear to be way dumber than Johnny, due to the accident that he himself caused
- Her slate blue moon hairclip is styled her very curly red hair, like a frizzy ponytail. This was shown in Johnny Escape From Bling Bling Island.
- She and Johnny both wear baggy jeans, except Mary's are slate blue, while Johnny's are evergreen.
- Mary is generally the most well liked character in the fandom due to her being more level-headed than any of the other characters.
- So far in the series, Mary has never 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 Brittney 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 entire break in production for MLP.
- In Season 7, Mary's will be voiced by Emily Tennant.
- In Johnny's Treasure, it is stated 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. Quantum of Johnny and Future Johnny are the only episodes she does not appear in .
- Mary's name is inspired by the word moon as moon starts with the letter m, her hair looks like a crescent moon, she wears a moon-shaped hairclip and a shirt with a moon on it.