Mrs. Lonstein is the mother of Schmuely "Snot" Lonstein and is seen in "Moon Over Isla Island". When Snot accidentally get a face full of Francine's bosom, Steve tries to get even but gets more than he bargained for when Mrs. Lonstein accidentally brushes his crotch.
Mrs. Lonstein is referenced twice in "The Magnificent Steven"; Snot accuses Steve, Barry and Toshi of being creeped out by his mother's wooden foot, and Steve mentions that Snot's father shot his mother in the face with a flare gun in the past.
Fed up with Snot's badgering him over his Animal Midwife game in "Don't Look a Smith Horse in the Mouth", Steve suggests they go to his house and watch Mrs. Lonstein try to sober up before going to her nursing job. She also sold Snot's costume in "Best Little Horror House in Langley Falls" to buy dental dams to keep from spreading disease to her lovers. Steve tries to get Snot to look on the bright side of spending all of his money on gifts for Hayley in "Jenny Fromdabloc" by pointing out that his mom couldn't steal it from his account any more.
Snot's cover story for leaving his home and Steve in "Why Can't We Be Friends?" is that his family moved to Wyoming because his mom liked the cowboy license plates. Meanwhile, Stan sends her a fake message that he has left for training to become an astronaut doctor.
Snot reveals that his father ran away from his family five years previously on a trip to Home Depot in "The Adventures of Twill Ongenbone And His Boy Jabari".
Snot admits he was a formula baby in "Steve and Snot's Test-Tubular Adventure" because his mom is an alcoholic.
Mrs. Lonstein has affairs with many of the characters, including Greg Corbin, Principal Lewis and Stelio Kontos in "Criss-Cross Applesauce: The Ballad Of Billy Jesusworth".
She is selected by Snot to spend the day at a water park with him over Steve in "Roger's Baby".
Mrs. Lonstein is voiced by Susie Essman.