Alcott was the daughter of noted transcendentalist and educator Amos Bronson Alcott and Abigail May Alcott. She shared a birthday with her father on November 29, 1832. In a letter to his brother-in-law, Samuel Joseph May, a noted abolitionist, her father wrote: "It is with great pleasure that I announce to you the birth of my second daughter...born about half-past 12 this morning, on my [33rd] birthday." Though of New England heritage, she was born in Germantown, which is currently part of Philadelphia, Pennsylvania. She was the second of four daughters: Anna Bronson Alcott was the eldest; Elizabeth Sewall Alcott and Abigail May Alcott were the two youngest. The family moved to Boston in 1834, After the family moved to Massachusetts, Alcott's father established an experimental school and joined the Transcendental Club with Ralph Waldo Emerson and Henry David Thoreau.

In 1840, after several setbacks with the school, the Alcott family moved to a cottage on of land, situated along the Sudbury River in Concord, Massachusetts. The Alcott family moved to the Utopian Fruitlands community for a brief interval in 1843-1844 and then, after its collapse, to rented rooms and finally to a house in Concord purchased with her mother's inheritance and financial help from Emerson. They moved into the home they named "Hillside" on April 1, 1845.

Alcott's early education included lessons from the naturalist Henry David Thoreau. She received the majority of her schooling from her father. She received some instruction also from writers and educators such as Ralph Waldo Emerson, Nathaniel Hawthorne, and Margaret Fuller, who were all family friends. She later described these early years in a newspaper sketch entitled "Transcendental Wild Oats". The sketch was reprinted in the volume Silver Pitchers (1876), which relates the family's experiment in "plain living and high thinking" at Fruitlands.

As an adult, Alcott was an abolitionist and a feminist. In 1847, the family housed a fugitive slave for one week. In 1848, Alcott read and admired the "Declaration of Sentiments" published by the Seneca Falls Convention on women's rights.

Poverty made it necessary for Alcott to go to work at an early age as an occasional teacher, seamstress, governess, domestic helper, and writer. Her first book was Flower Fables (1849), a selection of tales originally written for Ellen Emerson, daughter of Ralph Waldo Emerson. In 1860, Alcott began writing for the Atlantic Monthly. When the American Civil War broke out, she served as a nurse in the Union Hospital at Georgetown, D.C., for six weeks in 1862-1863. Her letters home – revised and published in the Commonwealth and collected as Hospital Sketches (1863, republished with additions in 1869) – garnered her first critical recognition for her observations and humor. Her novel Moods (1864), based on her own experience, was also promising.

She also wrote passionate, fiery novels and sensational stories under the nom de plume A. M. Barnard. Among these are A Long Fatal Love Chase and Pauline's Passion and Punishment. Her protagonists for these tales are willful and relentless in their pursuit of their own aims, which often include revenge on those who have humiliated or thwarted them. Written in a style which was wildly popular at the time, these works achieved immediate commercial success.

Alcott produced wholesome stories for children also, and after their positive reception, she did not generally return to creating works for adults. Adult-oriented exceptions include the anonymous novelette A Modern Mephistopheles (1875), which attracted suspicion that it was written by Julian Hawthorne; and the semi-autobiographical tale Work (1873).

Alcott's literary success arrived with the publication by the Roberts Brothers of the first part of Little Women: or Meg, Jo, Beth and Amy, (1868) a semi-autobiographical account of her childhood with her sisters in Concord, Massachusetts. Part two, or Part Second, also known as Good Wives, (1869) followed the March sisters into adulthood and their respective marriages. Little Men (1871) detailed Jo's life at the Plumfield School that she founded with her husband Professor Bhaer at the conclusion of Part Two of Little Women. Jo's Boys (1886) completed the "March Family Saga."

In "Little Women," Alcott based her heroine "Jo" on herself. But whereas Jo marries at the end of the story, Alcott remained single throughout her life. She explained her "spinsterhood" in an interview with Louise Chandler Moulton, "... because I have fallen in love with so many pretty girls and never once the least bit with any man." However, Alcott's romance while in Europe with Ladislas Wisniewski, "Laddie," was detailed in her journals but then deleted by Alcott herself before her death. Alcott identified Laddie as the model for Laurie in Little Women, and there is strong evidence this was the significant emotional and likely sexual relationship of her life.

In 1879 her younger sister, May, died. Alcott took in May's daughter, Louisa May Nieriker ("Lulu"), who was two years old. The baby had been named after her aunt, but was nicknamed Lulu, whereas Louisa May's nicknames were "Weed" and "Louy."

In her later life, Alcott became an advocate for women's suffrage and was the first woman to register to vote in Concord, Massachusetts, in a school board election.

Alcott, along with Elizabeth Stoddard, Rebecca Harding Davis, Anne Moncure Crane, and others, were part of a group of female authors during the Gilded Age who addressed women’s issues in a modern and candid manner. Their works were, as one newspaper columnist of the period commented, "among the decided 'signs of the times'" (“Review 2 — No Title” from The Radical, May 1868, see References below).

Alcott, who continued to write until her death, suffered chronic health problems in her later years. She and her earliest biographers attributed her illness and death to mercury poisoning: during her American Civil War service, Alcott contracted typhoid fever and was treated with a compound containing mercury. Recent analysis of Alcott's illness suggests that mercury poisoning was not the culprit. Alcott's chronic health problems may be associated with an autoimmune disease, not acute mercury exposure. Moreover, a late portrait of Alcott shows on her cheeks rashes characteristic of lupus. Alcott died of a stroke in Boston, on March 6, 1888, at age 55, two days after visiting her father's deathbed. Her last words were "Is it not meningitis?"

The story of her life and career was told initially in Ednah D. Cheney's Louisa May Alcott: Her Life, Letters and Journals (Boston, 1889) and then in Madeleine B. Stern's seminal biography Louisa May Alcott (University of Oklahoma Press, 1950). In 2008, John Matteson won the Pulitzer Prize in Biography for his first book, Eden's Outcasts: The Story of Louisa May Alcott and Her Father. Harriet Reisen's biography, "Louisa May Alcott: The Woman Behind Little Women," was published in 2009, and includes the most extensive primary source material (much discovered since Stern's biography), including Madelon Bedell's unpublished notes of interviews with Lulu before Lulu's death. The children's biography Invincible Louisa written by Cornelia Meigs received the Newbery Award in 1934 for the most distinguished contribution to American literature for children.

• The Inheritance (1849, unpublished until 1997)
• Flower Fables (1849)
• Hospital Sketches (1863)
• The Rose Family: A Fairy Tale (1864)
• Moods (1865, revised 1882)
• Morning-Glories and Other Stories (1867)
• The Mysterious Key and What It Opened (1867)
• Little Women or Meg, Jo, Beth and Amy (1868)
• Three Proverb Stories (includes "Kitty's Class Day", "Aunt Kipp" and "Psyche's Art") (1868)
• A Strange Island, (1868)
• Part Second of Little Women, also known as "Good Wives" (1869)
• Perilous Play, (1869)
• An Old Fashioned Girl (1870)
• Will's Wonder Book (1870)
• Aunt Jo's Scrap-Bag (1872—1882)
• Little Men: Life at Plumfield with Jo's Boys (1871)
• "Transcendental Wild Oats" (1873)
• A Story of Experience (1873)
• Eight Cousins or The Aunt-Hill (1875)
• Beginning Again, Being a Continuation of Work (1875)
• Silver Pitchers, and Independence: A Centennial Love Story," (1876)

• Rose in Bloom: A Sequel to Eight Cousins (1876)
• Under the Lilacs (1878)
• A Village Story (1880)
• The Candy Country (1885)
• Jo's Boys and How They Turned Out: A Sequel to "Little Men" (1886)
• Lulu's Library (1886—1889)
• A Garland for Girls (1888)
• Comic Tragedies (1893 [posthumously])

As A. M. Barnard

• Behind a Mask, or a Woman's Power (1866)
• The Abbot's Ghost, or Maurice Treherne's Temptation (1867)
• A Long Fatal Love Chase (1866 - first published 1995)

First published anonymously

• A Modern Mephistopheles (1877)