Born in Cheshire, England, she was educated in Lisieux, France and attended the Queen's University of Belfast, and studied at the Sorbonne in Paris, France. Her younger brother, Thomas Nesbitt, Jr., acted in one film in 1925, before his death in South Africa in 1927 from an apparent heart attack.

Her debut on the London stage was in the revival of Pinero's The Cabinet Minister (1910). She acted in countless plays after that.

In 1911, Nesbitt joined the Irish Players, went to the United States and debuted on Broadway in The Well of the Saints. She also was in the cast of John Millington Synge's The Playboy of the Western World with the Players when the whole cast was pelted with fruits and vegetables by the offended Irish American Catholic audience.

She became the love of English poet Rupert Brooke in 1912, to whom he wrote great love sonnets. They were engaged when he died during World War I. Nesbitt returned to the U.S. and appeared on Broadway in Quinneys (1915). After five other plays there, she returned to England.

Her film debut was in the silent A Star Over Night (1919). She then performed in The Faithful Heart (1922). She did not appear in a film again until 1930, when she played the role of Anne Lymes in Canaries Sometimes Sing, which was an early talkie.

She had one husband, actor Cecil Ramage. They married in 1920 and remained legally married until Nesbitt's death in 1982 but were separated for many years. They had two children.

Nesbitt's first Hollywood film was Three Coins in the Fountain (1954), in which she played the character role of La Principessa. This was followed that same year by Black Widow, in which she played a maid named Lucia Colletti.

Her other Broadway productions included Gigi (1951) starring Audrey Hepburn, Sabrina Fair (1953) starring Joseph Cotten and Margaret Sullavan, and Anastasia (1954). In 1956, she played Mrs. Higgins in the hit My Fair Lady starring Julie Andrews and Rex Harrison. Nesbitt, amazingly, played the role again in 1981!! in her in 90s in a Broadway revival of My Fair Lady also starring Rex Harrison, who had also appeared in the original Broadway show (and the film).

She is probably best-remembered for her role as Agatha Morley on the TV series The Farmer's Daughter from 1963 to 1966, playing the mother of a Congressman (played by William Windom). She guest starred on such shows as The United States Steel Hour; Wagon Train; Naked City, Dr. Kildare and Upstairs, Downstairs.

Nesbitt won an Emmy Award for her work in the TV drama The Mask of Love (1974).

Nesbitt lived for many years in the United States, and considered taking out U.S. citizenship, but ultimately returned to the United Kingdom, where she was awarded the CBE. Her autobiography, A Little Love and Good Company, was published in 1973.

She played the film role of an elderly drug addict in The French Connection II (1975). Her next film was Hitchcock's Family Plot (1976), in which she played Julia Rainbird. She then appeared as the grandmother in Julia (1977). Her final film was Never Never Land (1980), in which she played Edith Forbes.

After a career spanning over eighty years, one of the longest in show business history, Cathleen Nesbitt died at age ninety-three in London.