She is ancient compared to just about anyone in Bikini Bottom. She even remembers the time when chocolate was invented. She is shriveled up to the point where she appears to be merely a head and a spine. She has a severe hearing disability and no physical eyeballs left.
She is often bad-tempered and impatient, often irritating her daughter. Due to having a hearing disability, she has to have people repeat themselves for her a number of times and with a tone of voice as loud as possible. She despises chocolate, though develops an interest in it after being falsely told by SpongeBob and Patrick that rubbing it on oneself causes the user to live forever. From this instance, it is revealed that she wants to be immortal.
When SpongeBob and Patrick come to Mary's doorstep, her mother comes forth to see what they are selling. Since she cannot hear, Mary is forced to scream at the top of her lungs that they are selling chocolate bars, after which her mother reminisces on when chocolate was invented and how much she hated it. SpongeBob and Patrick, conforming to their profit gimmick, falsely advertise the chocolate they're selling as skin enhancers that lead their customers into a state of immortality. This changes her opinion on chocolate and she agrees to buy a bar, intent on living forever. Mary then says "I hate you" to SpongeBob and Patrick.
At the end of the episode, Mary and her mother double-date Patrick and SpongeBob at Fancy! after they become rich from selling their all their chocolate to Tom.
As the Bikini Bottomites are looking for a place to throw up after realizing that Squidward's ink is used to make black lemonade, Mary can be seen pushing her mother through the crowd of people going into Squidward's house.
Mary tricks SpongeBob and Patrick into babysitting Mary's mother. At first, they think she is an "ugly baby" that can talk. She kept trying to escape from them numerous times until Patrick accidentally mistakes her for a worm and lets her out. SpongeBob then realizes that she is really a 137-year-old lady so he and Patrick go search for her while she does extreme activities such as snowboarding on lava, skydiving without a parachute, and boxing. When SpongeBob and Patrick finally find her, she beats them up due to them humiliating her earlier. She then tires herself out and SpongeBob and Patrick take her home.
In "Biddy Sitting," she is revealed to have an old birthday card from her 137th birthday. She is pictured on the card and looks noticeably younger, with a body instead of just a spine. This means she is older than 137. Her exact age is unknown.
If she was alive to see the invention of chocolate, she would have been born before 1847.
She must've drank Patrick's Ink Lemonade if she was shown dashing to Squidward's house to vomit.