Fiennes was born in Ipswich, Suffolk, England, the son of Mark Fiennes (1933-2004), a farmer and photographer, and Jennifer Lash (1938-1993), a writer. His surname is of Norman origin. He is a third cousin of the adventurer Sir Ranulph Fiennes. The eldest of six children, his siblings are actor Joseph Fiennes (Shakespeare in Love, Luther), Martha Fiennes, a director (in her film Onegin, he acted the title role), Magnus Fiennes, a composer, Sophie Fiennes, a filmmaker, Jacob Fiennes, a conservationist, and his foster brother Michael Emery, an archaeologist. The Fiennes family moved to Ireland in 1973, living in West Cork and County Kilkenny for some years, where Fiennes attended Saint Kieran's College for one year. It has been alleged by locals in Kilkenny that this year was a thoroughly unenjoyable one. They moved to Salisbury in England, where Fiennes finished his schooling at Bishop Wordsworth's School before attending Chelsea College of Art.[citation needed]

Fiennes trained at the Royal Academy of Dramatic Art and then joined the Royal Shakespeare Company in 1988. He is the only actor ever to have won a Tony Award for playing Hamlet on Broadway. In 2001, Fiennes received the William Shakespeare Award from the Shakespeare Theatre in Washington, D.C. Fiennes made his film debut in 1992 as Heathcliff in Wuthering Heights opposite Juliette Binoche, for which he received substantial acclaim and praise throughout Europe. 1993 was the "breakout year" for him. He had a major role in the very controversial Peter Greenaway film The Baby of Mâcon with Julia Ormond. Though the film was poorly received, Fiennes suffered no lasting consequences, as later that year he became known internationally for portraying the amoral Nazi concentration camp commandant Amon Goeth in Steven Spielberg's Schindler's List. For this he was nominated for the Academy Award for Best Supporting Actor. He did not win the Oscar, but did win the Best Supporting Actor BAFTA Award for the role. His portrayal as Göth also earned him a spot on the American Film Institute's Top 50 of movie villains. In 1994, he portrayed American academic Charles Van Doren in Quiz Show, and in 1996 was nominated for the Academy Award for Best Actor for the World War II epic romance The English Patient. Fiennes' work has ranged from thrillers (Red Dragon) to animated Biblical epic (The Prince of Egypt) to campy nostalgia (The Avengers) to romantic comedy (Maid in Manhattan) and offbeat dramedy (Oscar and Lucinda). Fiennes was cast as Lord Voldemort in the 2005 fantasy film Harry Potter and the Goblet of Fire and has retained this role for the 5th film Harry Potter and the Order of the Phoenix. The Constant Gardener was also released in 2005, with Fiennes as the title role. The film is set in the slums of Kibera and Loiyangalani, Kenya. The situation affected the crew to the extent that they set up the Constant Gardener Trust in order to provide basic education around these villages. Fiennes is a patron of the charity. His recent performance in the play Faith Healer gained him a nomination for a 2006 Tony Award.

Fiennes is a UNICEF ambassador. Fiennes met actress Alex Kingston (Dr. Corday from ER) while both were students at the Royal Academy of Dramatic Arts. After dating for ten years, they married in 1993. However, they divorced in 1997. In 1995, Fiennes started dating Francesca Annis, an actress 17 years his senior, who played his mother in Hamlet. In February 2006 the couple separated. This came after tabloid reports that said Fiennes had had an affair with Romanian singer Cornelia Crisan. In late 2006, sources reported that Fiennes was dating American actress Ellen Barkin.