Mackintosh has a sister, Fiona Mackintosh. He trained at the Sylvia Young Theatre School and began his career as a child actor. He is married to the actress Lisa Jacobs and the couple have two children, Martha and Blythe.

Mackintosh first became generally well known for his appearances in the television show The Bill.

He has had major roles in A Dark-Adapted Eye (1994), Prime Suspect 5 (1996), Care (2000), The Other Boleyn Girl (2003), as well as the lead role in the BBC adaptation of Dickens' Our Mutual Friend. In the 2006 BBC drama series The Amazing Mrs Pritchard, he played Mrs Pritchard's husband Ian.

Mackintosh is probably best known for his role in the British film Lock, Stock and Two Smoking Barrels (1998), but has appeared in numerous other films, including Princess Caraboo (1994) and Underworld: Evolution (2006).

He played the role of a British officer, McKinley, in the hit Bollywood movie Rang De Basanti in 2006.