Better known as Fanny Fern, Sara was born in Portland, Maine, and was originally named Grata Payson after the mother of the Congregational minister Edward Payson, whose sermons had influenced her father a great deal. She was the fifth of nine children born to Nathaniel and Hannah (Parker) Willis. In the early 17th century, her predecessors had come from England to Massachusetts. Her paternal grandfather edited a Whig journal in Boston during the revolutionary war, and her father published an anti-federalist organ, the Eastern Argus in Portland. A libel suit over his excessive religious zealousness found him returned to Boston the year after Sara's birth. There he started a printing business, and in 1816 established the Boston Recorder, one of America's first religious newspapers. However, he is mostly remembered as the founder of the Youth's Companion in 1827. Referring to her mother Sara once wrote "she made everyone who came near her better and happier." Her brother Nathaniel became famous as a poet an editor of the New York Mirror. Another brother Richard, was a composer and editor of the Musical World and Times.

Her school girl personality earned her the nickname "Sal Volatile." Her classmates at the seminary of Catherine Beecher in Hartford, Connecticut were always impressed with her witty personality. After she returned to Boston, she often contributed to her father's Youth's Companion. A well to do bank cashier named Charles H. Eldredge became her husband in May, 1837. Together they had three children, Mary in 1839, Grace Harrington in 1840, and Ellen Willis in 1844. This wonderful time in her life was ended by the death of her mother, followed by the death of her daughter Mary, and in 1846 she became a widow. With little help or financial support from her father and the Eldredges, unable to find work, in 1849 she married Samuel P. Farrington, a widower. This marriage ended three years later in divorce. Once again on her own, she attempted teaching and sewing. However she made so little that she had to give up her daughter Grace to the Eldredge grandparents. Finally, she decided to write and used the pseudonym "Fanny Fern." The Mother's Assistant, the True Flag, and the Olive Branch, small Boston magazines, published some of her work in 1851. Before long, many newspapers reprinted these amusing paragraphs.

Her journalistic career now on its way, garnered the attention of publisher James C. Derby from Auburn, New York. He gathered her writings together and published them in 1853 as Fern Leaves from Fanny's Port-Folio. It became an immediate bestseller in both America and England. In 1854, a second series of Fern Leaves was published as well as Little Ferns for Fanny's Little Friends, a book for children. The three books combined sold 132,000 copies in United States, and 48,000 abroad. Over $10,000 in royalties was received in less than two years. Robert Bonner, owner of the New York Ledger, placed her on his payroll in 1855 at the astounding salary of $100 for a regular weekly column. This made her one of America's first women columnists. She moved to New York, and worked for The Ledger for the remainder of her life, never missing an issue and reaching a weekly audience of half a million readers. During the next 15 years, a series of pieces from The Ledger were published in seven different books, two of them for children. She also wrote two novels, Ruth Hall (1855) and Rose Clark (1856).

A large woman, with light brown hair, florid complexion, and large, blue eyes, it was said candidly of her by another that "Fanny isn't handsome, and never was," but was also admitted that she had "a splendid form, a charming foot and ankle, a fascinating expression, and the manners of a Queen." Her literature revealed popular tastes, and as social history, cast a wonderful light on American home life of the era. Her pieces were chatty, spicy, witty, and impudent, yet they were sympathetic towards domestic problems in a way which women could understand. She promoted intellectual equality between the sexes, pooh-pooh'd the moral double standards of the time, deplored too much house work and families that were too large, and suggested that women seek wider fields of interest in their lives, while at the same time she poked fun at the male chauvinist. With a love of children, she encouraged parents to respect each child's individuality, and had special empathy for the willful and the tomboy. She kept herself away from the suffrage movement, saying it was "better policy to play opossum, and wear the mask of submission." By 1858, she was supporting the movement. In 1868, together with Jane C. Croly, she became one of the founders of New York City's pioneer woman's clubs, Sorosis.

She married again on Jan. 5, 1856, biographer James Parton even though she was 11 years older. Her daughter Ellen lived with them. The marriage got off to a rocky start, however, a new baby, Ethel, born to Fanny's daughter Grace, who died in 1861, seemed to bring things together. Sarah was stricken with cancer, and at the age of 61, lost her six-year battle and died in New York City in 1872. Her body was taken to Cambridge, Massachusetts, where she was buried in Mount Auburn cemetery. In 1876, her daughter Ellen became James Parton's second wife.