Jessica Lucy "Decca" Treuhaft (née Freeman-Mitford, later Romilly; 11 September 1917 – 23 July 1996) was an English author and activist, one of the Mitford sisters: six girls born to David Freeman-Mitford, 2nd Baron Redesdale and his wife Sydney Bowles (1880–1963), namely Nancy (1904–1973), Pamela (1907–1994), Diana (1910–2003), Unity (1914–1948), Jessica, and Deborah (1920–2014). The sisters had one brother Tom who was killed in action in 1945.

Jessica married her second cousin Esmond Romilly, emigrating to the United States in 1939. Romilly was reported missing in action in 1941, and in 1943 Jessica married American civil rights lawyer Robert Treuhaft, with whom she worked closely in the Civil Rights Congress. She took American citizenship in 1944. The couple joined the Communist Party USA, and both refused to testify in front of the House Un-American Activities Committee.

Her 1960 memoir Hons and Rebels and her 1963 book of social commentary The American Way of Death both became classics.

Born at Asthall Manor, Oxfordshire, the sixth of seven children, Jessica Mitford was the daughter of David Freeman-Mitford, 2nd Baron Redesdale, and his wife Sydney (daughter of politician and publisher Thomas Bowles). Refused permission by her parents to attend school, Jessica was educated at home by governesses, and increasingly rebelled against her heavily circumscribed home life. When she was twelve she opened a 'running away' account at Drummond's Bank, and her social and political conscience was awakened as she read newspaper accounts of the economic depression facing the Britain of the late 1920s.

Jessica's older sister, Unity, with whom as a girl she shared a bedroom, had beliefs at the opposite end of the political spectrum, later becoming a dedicated Nazi. The two drew a chalk line down the middle to divide the room. Jessica's side was decorated with hammer and sickles and pictures of Vladimir Lenin, while Unity's was decorated with swastikas and pictures of Adolf Hitler.

Jessica described her conservative father as "one of nature's fascists", renounced her privileged background while still a teenager, and became an adherent of communism. Mitford said that her parents had "appeased Hitler and Nazism. ... He had crushed the trade unions, he had crushed the Communist Party and he had crushed the Jews ... and don't forget there's a huge strain of anti-Semitism that runs through that class in England." She has been called the "red sheep" of the family.

At the age of 19, Mitford fell in love with her second cousin, Esmond Romilly, who was recuperating from dysentery caught while defending Madrid with the International Brigades during the Spanish Civil War. Romilly was a nephew (by marriage) of Winston Churchill. The cousins eloped to Spain, where Romilly picked up work as a reporter for the News Chronicle. After some legal difficulties caused by their relatives' opposition, they married. They moved to London and lived in the East End, then mostly a poor industrial area. Mitford gave birth at home to a daughter, Julia Decca Romilly, on 20 December 1937. The baby died in a measles epidemic the following May. Jessica Mitford rarely spoke of Julia in later life, and she is not referred to by name in Mitford's 1960 autobiography, Hons and Rebels.

In 1939, Romilly and Mitford emigrated to the United States. They travelled around, working odd jobs. At the outset of World War II, Romilly enlisted in the Royal Canadian Air Force; Mitford was living in Washington D.C., and considered joining him once he was posted to England. While living in D.C, with contemporaries Virginia Foster Durr and Clifford Durr, she gave birth to another daughter, Constancia Romilly ("the Donk" or "Dinky") on 9 February 1941. Her husband went missing in action on 30 November 1941, on his way back from a bombing raid over Nazi Germany. Mitford was devastated.