Sally Mueller is a resident girl of West Broadway, the wealthiest part of Derry, and Greta Bowie’s best friend. She is also a classmate of the Losers’ Club in Ms. Douglas class. Her family and ancestors are well known for being part of Derry's incidents as well as milestones and events. Her uncle was part of the group of people who burned down a bar known as "Black Spot". It is not known what happens to her after the summer of 1958.
She first appears in Ben Hanscom’s flashback on the last day of 5th grade. During Ms. Douglas’ class, Sally tells Beverly Marsh she hopes that she passes but later on looks at Beverly Marsh in disgust after she sweared. Ben describing her and Greta Bowie as a pair of stuck-up snobs due to their rich families, nice clothes and permed hair.
Later on in the summer, Richie Tozier and Bill Denbrough sit on the stone steps of her family’s house which is described as a “rambling red Victorian structure”.
Soon after Richie comes across Beverly Marsh and mentions how pretty she is and soon says that Greta and Sally loathe Beverly for being extremely poor and for being prettier then them. Beverly and Richie begin to make fun of Marcia Fadden, Sally Mueller and Greta Bowie saying that they “pee rose water”, which is their way of saying they are unlikable rich kids.
She is later mentioned to thought be the daughter of a man that has the same last name as her, but is really her uncle.
On the night of Beverly’s encounter with IT, Beverly remembers Greta and Sally talking about sex-act.
Later on in 1985, Eddie Kaspbrak walks down West Broadway which was the wealthy parts of Derry and where Sally grew up. As Eddie walked down the street of Victorian houses, he comes across The Mueller’s old house which is described as a red Victorian house with squared off turrets on each side and hedges in the front yard that sits on the corner of Witcham Street and West Broadway. The Mueller’s are mentioned to have a gardener that glares strangely at Eddie when ever he walks by.
In the book, Sally is described as to wearing nice clothes like her friend Greta Bowie and is said to have her hair permed every month. In the film adaption, she wears a field hockey uniform and has brunette hair pulled back in a ponytail.
Though absent in the miniseries, Sally Mueller appears in the 2017 film adaptation, her character portrayed by Jocelyn Mattka. Just as in the book, she is the best friend of Greta Bowie and is also close friends with Marcia Fadden. Sally, along with Marcia, appears early yet briefly in the movie, where the two girls join Greta in bullying Beverly Marsh in the bathroom, Sally being the one who points out to the newly arriving Greta which bathroom stall Beverly is inside.