Harriett Mulford Stone was born in New Haven, Connecticut. in 1844.

In 1878, at the age of 34, she began sending short stories to Wide Awake, a children's magazine in Boston. Two of her stories, "Polly Pepper's Chicken Pie" and "Phronsie Pepper's New Shoes", proved to be very popular with readers. Daniel Lothrop, the editor of the magazine, requested that Stone write more.

The success of Harriett's short stories prompted her to write the now-famous Five Little Peppers series. This series was first published in 1881, the year that Stone married Daniel Lothrop. Daniel had founded the D. Lothrop Company of Boston, who published Harriett's books under her pseudonym, Margaret Sidney.

Harriett and Daniel may have both had an interest in history and in famous authors. In 1883, they purchased the house in which both Louisa May Alcott and Nathaniel Hawthorne had lived. Nicknamed The Wayside, the house is located in Concord, Massachusetts. The year after Harriett and Daniel moved into the house, Harriett gave birth to their daughter, Margaret, at the age of 40.

Daniel Lothrop died on Friday, March 18, 1892, when Harriett was 48 and their daughter was just 9 years old. There was a gap in the release of the Five Little Peppers books from 1892 to 1897, while Harriett continued to run the publishing company Daniel founded. Eventually, she sold the company, which later became Lothrop, Lee & Shepard Co. It continued to publish Harriett's books under the Margaret Sidney name when Harriett resumed writing the Five Little Peppers series.

Harriett eventually wrote over 30 books; in addition to the Five Little Peppers series she wrote a number of books on patriotic themes, including A Little Maid of Concord Town and A Little Maid of Boston. She died at the age of 80.

Harriett was involved with Daughters of the American Revolution. In 1895, Harriett formed a children's group of a similar vein called Children of the American Revolution. This group is still around today, and was organized "for the training of young people in true patriotism and love of country".

She loved travelling overseas, but spent many winters in California where the climate was more agreeable.

Alongside her writing career, Stone had a deep interest in historical homes and buildings and worked hard to preserve them. These include:

• The Wayside, where she lived with her family
• Orchard House, which belonged to the Alcott family and was next door to the Wayside
• Grapevine Cottage, where the Concord grape was first produced
• The Tolman House in Dorchester, Massachusetts built during the colonial period

In later years, Stone's daughter Margaret Lothrop championed to have her childhood home, The Wayside, declared a National Historic Landmark. It was made so in 1963.