Polly Milton, a bright 14-year-old country girl, visits her friend Fanny Shaw and her wealthy family in the city for the first time. Poor Polly is overwhelmed by the splendor at the Shaws' and their urbanized, fashionable lifestyles, expensive clothes and other habits she has never been exposed to, and, for the most part, dislikes. Fanny's friends ignore her because of her different behavior and simple clothing, Fanny's brother Tom teases her, and Fanny herself can't help considering her unusual sometimes. Despite feeling odd and out of place, and amid the new fashions and the constant demands of the Shaws, Polly keeps on being kind to the Shaw family, and eventually, Polly's warmth, support, and kindness eventually win the hearts of all the family members, and her old-fashioned ways teach them a lesson they will never forget. Polly grows closer to Fanny, and creates strong bonds with Tom and his younger sister, Maud. After six weeks with the Shaws, Polly returns to her country home.

Over the next six years, Polly visits the Shaws every year and comes to be considered a member of the family. Later, Polly comes back to the city to become a music teacher. She learns that Tom is engaged to Fanny's bratty friend, Trix. Polly rents a room in the home of an elderly lady named Miss Mills, who tells her about a teenage girl named Jane who attempted suicide. Polly takes Jane under her wing and shows her that life is truly worth living.

Later, Polly goes to the opera with Fanny and Tom. They happen to meet Mr. Sydney, a friend of the Shaws. Polly, described as a raving beauty by Tom, never plays up her looks and normally dresses simply. On this night, however, she dresses up in Fanny’s borrowed finery, and her beauty shines. Mr. Sydney and Tom both take notice of Polly, and, enjoying the attention, Polly innocently flirts with both of them. Mr. Sydney becomes attracted to Polly, and lonely Polly enjoys the attention, and appreciates Mr. Sydney’s friendship. He begins to learn her routine and “coincidentally” runs into her and accompanies her during her walks to her students’ homes.

When Fanny becomes suddenly gloomy one day, Polly understands that, since Fanny became a young woman so early in her life, she was already tired of life. She takes Fanny to visit her friends, Becky, Bess, and Kate, the latter happening to be an author Fanny is fond of. Becky is carving a statue that, she tells the other women, embodies feminism.

Polly comes to understand that Fanny is attracted to Mr. Sydney, and is jealous of Mr. Sydney’s growing attraction to Polly. Polly feels guilty for unintentionally encouraging Mr. Sydney, and unwittingly causing Fanny pain, and determines to nip his attraction in the bud.

Later in the book, Tom tells Polly that he is in great debt. Polly persuades him to tell his father. When he does, his father tells him that they are on the brink of bankruptcy. As the Shaws' lives change drastically, Polly guides them to the realization that wholesome family life is the only thing they will ever need, not money or decoration. Mr. Sydney later gives Tom the money to pay his debts.

With the comfort of the ever-helpful Polly, the family gets to change for the better and to find a happier life for all of them. After Trix calls off their engagement, Tom procures a job out West, with Polly's brother Ned, and heads off to help his family and compensate for all the money he has wasted in frivolous expenditures. Throughout the year he is gone, Fanny strikes up a relationship with Mr. Sydney, and the two get engaged. A year later, Tom returns from the West and gets engaged to Polly, his longtime love.

The novel was the basis of a 1949 musical film starring Gloria Jean as Polly.

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•   Works related to An Old Fashioned Girl at Wikisource
• An Old-Fashioned Girl (c1870) at A Celebration of Women Writers
•   An Old-Fashioned Girl public domain audiobook at LibriVox
• An Old-Fashioned Girl at IMDb