Firth was born in Grayshott, Hampshire, England to David (a history lecturer and the son of an Anglican priest) and Shirley (a comparative religion lecturer, whose own parents were Congregationalist ministers); he has a sister, Kate, and a younger brother, Jonathan, who is now also an actor. Both of Firth’s parents were born and raised in India, as their English-born parents were involved in missionary work.

Firth spent part of his childhood in Nigeria, where his father was teaching. He lived in St. Louis, Missouri when he was 11. He later attended the Montgomery of Alamein Secondary School, a state comprehensive school in Winchester, Hampshire, and then Barton Peveril College in Eastleigh, Hampshire.

In 1989, Firth entered into a romantic relationship with actress Meg Tilly, his co-star in Valmont. In 1990, she gave birth to a son, Will Firth.

In 1994, Firth was involved with actress Jennifer Ehle, his co-star in Pride and Prejudice. Currently, Firth lives both in London and Italy. He is married to Italian documentary filmmaker Livia Giuggioli (since 21 June 1997). The couple have two sons, Luca (born 2001) and Matteo (born 2003).