He is best known for his roles as Professor Nick Cutter in the science fiction series Primeval (2007–2011) and Detective Inspector Jimmy Pérez in the crime drama Shetland (2013–2022).

He later returned to London where he received critical acclaim for his theatre work, notably Life of Stuff at the Donmar Warehouse (1993) and American Buffalo at the Young Vic (1997).

One of his first successful film roles was as Edgar in Angels and Insects (1995) before going on to star in Sharpe's Justice (1997), Orphans (1998), The Man with Rain in His Shoes (1998).

He has also starred in many television series and is known for his roles in Psychos (1999) This Year's Love (1999) and Kid in the Corner (1999) (for which he won a gold nymph as best actor in a mini-series at the Monte-Carlo TV festival in 2000).

In the summer of 2002, Douglas returned to the London stage where he performed the role of Michael Bakunin in Tom Stoppard's new trilogy of plays, The Coast of Utopia, at the National Theatre.

In 2012, he starred as Augustus Cribben in The Secret of Crickley Hall, and in the ITV television film of Ian Rankin's novel Doors Open.

In 2016, he starred in the Scottish three-part television drama series In Plain Sight as the detective William Muncie, who pursued serial murderer Peter Manuel to his conviction and ultimate execution by hanging.

[11] In 2019 he appeared as antiquarian book dealer Adam Snow in Susan Hill's ghost story The Small Hand (Channel 5 TV).