Born in Lincolnshire, the son of Roy Broadbent, a furniture maker who turned a former church into a theatre named after him, and a sculptress (his twin sister died at birth), Broadbent was educated at Leighton Park School, a Quaker school in Reading, and briefly attended art college before transferring to the London Academy of Music and Dramatic Art. He graduated in 1972 and went on to work for the Royal National Theatre and the Royal Shakespeare Company as well as acting in Ken Campbell's epic Illuminatus (1976). He also founded the comedy group, the National Theatre of Brent with Patrick Barlow. Broadbent is also the honorary president of the Lindsey Rural Players whose home theatre is the Broadbent Theatre (link below).

Jim Broadbent made his film debut in 1978 with a tiny role in Jerzy Skolimowski's The Shout, and made his television debut the following year. He went on to work with Stephen Frears (for television, and in The Hit (1984)) and Terry Gilliam (in Time Bandits (1981) and Brazil (1985)) before establishing himself in Mike Leigh's Life is Sweet (1990). He proved his ability as a character actor in films including The Crying Game (1992), Enchanted April (1992), Bullets Over Broadway (1994), The Borrowers (1997) and Little Voice (1998) before taking a leading role in another Mike Leigh film, Topsy-Turvy (1999). 2001 proved to be a breakthrough year for Broadbent, who starred in three of the year's most successful films: Bridget Jones' Diary (2001), Moulin Rouge! (2001), for which he won a BAFTA and Iris (2001), for which he won an Oscar for his portrayal of John Bayley. He is regarded as one of cinema's most reliable character actors and has a reputation of being very easy to work with.

Broadbent is best known to television audiences as DCI Roy 'The Slag' Slater, an associate character in the enormously popular sitcom Only Fools and Horses. The character appeared in three episodes over an eight-year period. He had originally been offered the lead role of Del Trotter in the series, but he turned it down due to other commitments. Other comic roles include the lead role in the sitcom The Peter Principle and occasional guest appearances in Not The Nine O'Clock News.