Fiennes was born in Salisbury, Wiltshire, England, the son of novelist Jennifer Lash and photographer Mark Fiennes. The youngest of six siblings, he was raised in West Cork, Ireland.

He is the brother of actor Ralph Fiennes, film makers Sophie Fiennes and Martha Fiennes, composer Magnus Fiennes and, his twin, Jacob Fiennes, a conservationist. His foster brother, Michael Emery, is an archaeologist. He is a cousin of Sir Ranulph Fiennes and eighth cousin of the Prince of Wales.

After leaving art school, he worked with the Young Vic Youth Theatre. He continued his training at the Guildhall School of Music and Drama. His first professional stage appearance was in the West End in The Woman in Black, followed by A Month in the Country'' opposite Helen Mirren. Fiennes then became a member of the Royal Shakespeare Company for two seasons.

He made his television debut as Willy in The Vacillations of Poppy Carew. His first feature film was 1996's Stealing Beauty, co-starring Liv Tyler. In 1998, Fiennes appeared in two films that were nominated at the Academy Awards: he played Robert Dudley opposite Cate Blanchett in Elizabeth and he also portrayed William Shakespeare opposite Gwyneth Paltrow in his most popular film to date Shakespeare in Love, which won the award for Best Picture.

Following the success of Shakespeare in Love, Fiennes turned down a 5 picture deal from a major studio and withdrew from mainstream cinema. Instead he chose to return to the theatre. He remains highly selective of his work and has famously turned down major film productions in order to appear on stage.

Fiennes has also worked in other mediums. In 2003 he lent his voice as Proteus in the DreamWorks animated film Sinbad: Legend of the Seven Seas opposite Brad Pitt. The same year he starred in the limited-release film Luther, playing Martin Luther, and he also appeared in The Merchant of Venice , in which he portrayed Bassanio.

In 2006, he appeared in The Darwin Awards opposite Winona Ryder, and in Running With Scissors, once again teaming him up with Gwyneth Paltrow. Fiennes will return to the theatre again this year in the one-man play Unicorns, Almost about World War II poet Keith Douglas at the Old Vic .

In Spring 2006, he was in South Africa filming Goodbye Bafana with director Bille August.The film was released in 2007. Fiennes portrayed James Gregory, author of the book Goodbye Bafana: Nelson Mandela, My Prisoner, My Friend.