Garnett was born in Brighton, England, the sixth of the eight children of the solicitor David Black (1817–1892), afterwards town clerk and coroner, and his wife, Clara Maria Patten (1825–1875), daughter of George Patten. Her brother was the mathematician Arthur Black, and her sister was the labour organiser and novelist Clementina Black. Her father became paralysed in 1873, and two years later her mother died from a heart attack after lifting him from his chair to his bed.
She was initially educated at Brighton and Hove High School. Afterwards she studied Latin and Greek at Newnham College, Cambridge, on a government scholarship. In 1883 she moved to London, where she started work as a governess, and then as the librarian at the People's Palace Library. Through her sister, Clementina, she met Dr. Richard Garnett, then the Keeper of Printed Materials at the British Museum, and his son Edward Garnett, whom she married in Brighton on 31 August 1889. Edward, after working as a publisher's reader for T. Fisher Unwin, William Heinemann, and Duckworth, went on to become a distinguished reader for the publisher Jonathan Cape. In the summer of 1891, then pregnant with her only child, she was introduced by Edward to the Russian exile Feliks Volkhovsky, who began teaching her Russian. He also introduced her to his fellow exile and colleague Sergius Stepniak and his wife Fanny. Soon after, Garnett began working with Stepniak, translating Russian works for publication; her first published translations were "A Common Story" by Ivan Goncharov, and "The Kingdom of God is Within You" by Leo Tolstoy. The latter was published while she was making her first trip to Russia in early 1894. After visits to Moscow and Saint Petersburg, she travelled to Yasnaya Polyana where she met Tolstoy; although the latter expressed interest in having her translate more of his religious works, she had already begun working on the novels of Turgenev and continued with that on her return home. Initially she worked with Stepniak on her translations; after his untimely death in 1895, Stepniak's wife Fanny worked with her. From 1906, her favourite amanuensis was a Russian girl, Natalie Duddington whom she had met in Russia and in whom she found "real intellectual companionship".
Over the next four decades, Garnett would produce English-language versions of dozens of volumes by Tolstoy, Gogol, Goncharov, Dostoyevsky, Turgenev, Ostrovsky, Herzen and Chekhov.
Her son and only child, David Garnett, trained as a biologist and later wrote novels, including the popular Lady into Fox (1922).
By the late 1920s, Garnett was frail and half-blind. She retired from translating after the publication in 1934 of Three Plays by Turgenev. After her husband's death in 1937, she became quite reclusive. She developed a heart condition, with attendant breathlessness, and in her last years had to walk with crutches. She died at The Cearne, Crockham Hill, Kent, at the age of 84.
Constance Garnett translated 71 volumes of Russian literary works, and her translations received high acclaim, from authors such as Joseph Conrad and D. H. Lawrence. Despite some complaints about being outdated, her translations are still being reprinted today (most are now in the public domain).
However, Garnett also has had critics, notably prominent Russian natives and authors Vladimir Nabokov and Joseph Brodsky. Nabokov claimed that Garnett's translations were "dry and flat, and always unbearably demure." Nabokov's criticism of Garnett, however, may arguably be viewed in light of his publicly stated ideal that the translator be male. Brodsky notably criticised Garnett for blurring the distinctive authorial voices of different Russian authors:
"The reason English-speaking readers can barely tell the difference between Tolstoy and Dostoyevsky is that they aren't reading the prose of either one. They're reading Constance Garnett."
In her translations, she worked quickly, and smoothed over certain small portions for "readability", particularly in her translations of Dostoyevsky. In instances where she did not understand a word or phrase, she omitted that portion.
For his Norton Critical Edition of The Brothers Karamazov, Ralph Matlaw based his revised version on her translation. This is the basis for the influential A Karamazov Companion by Victor Terras. Matlaw published an earlier revision of Garnett's translation of the Grand Inquisitor chapter in a volume paired with Notes From Underground.
In 1994 Donald Rayfield compared Garnett's translations with the most recent scholarly versions of Chekhov's stories and concluded:
"While she makes elementary blunders, her care in unravelling difficult syntactical knots and her research on the right terms for Chekhov's many plants, birds and fish are impressive.... Her English is not only nearly contemporaneous to Chekhov's, it is often comparable."
Her translations of Turgenev were highly regarded by Rachel May, in her study on translating Russian classics.
Later translators such as Rosemary Edmonds and David Magarshack continued to use Garnett's translations as models for their own work.