Mark Strong is best known for his film roles such as Prince Septimus in Stardust (2007), Archibald in RocknRolla (2008), Lord Henry Blackwood in Sherlock Holmes (2009), Frank D'Amico in Kick-Ass (2010), Jim Prideaux in Tinker Tailor Soldier Spy (2011), Sinestro in Green Lantern (2011), George in Zero Dark Thirty (2012), Major General Stewart Menzies in The Imitation Game (2014), Merlin in Kingsman: The Secret Service (2014) and Kingsman: The Golden Circle (2017), Dr. Thaddeus Sivana in Shazam! (2019), and John in Cruella (2021).
At the beginning of the 2000s, Strong appeared in Heartlands and in Shakespeare's Twelfth Night at the Donmar Warehouse, for which he was nominated for the 2003 Laurence Olivier Award for Best Performance in a Supporting Role. He was later featured in Roman Polanski's Oliver Twist (2005), played Mussawi in the film Syriana (2005), and played an assassin named Sorter in Revolver (2005). He portrayed the traitorous Wictred in Tristan & Isolde, showing his talent with swordplay, and since 2006, he has provided the narration in the BBC's genealogy series Who Do You Think You Are?.
Strong played Clive Cornell in The Guard (2011), which is the most successful independent Irish film of all time in terms of Irish box-office receipts, overtaking The Wind that Shakes the Barley (2006). He next portrayed the role of Jacob Sternwood in the British thriller Welcome to the Punch. He played Matai Shang, the leader of the Therns in John Carter (2012), and starred in the action thriller film Zero Dark Thirty (2012). The film had its premiere in Los Angeles on 19 December 2012 and had its wide release on 11 January 2013. The film received wide critical acclaim and was nominated for five Academy Awards, including Best Picture.
In 2015, Strong won the Olivier Award for Best Actor for his role in A View from the Bridge, which he starred in as Eddie Carbone at the Young Vic. When Strong went over to perform the play on Broadway in New York, he received similar levels of acclaim, and was nominated for the Drama Desk Award for Outstanding Actor in a Play, and the Tony Award for Best Actor in a Play, for his performance.
In March 2020, Mark Strong narrated public information films for the UK government which outlined how the British public should approach daily life during the COVID-19 pandemic.
In March 2021, Strong was the castaway on BBC Radio 4's Desert Island Discs. Later that same year, he co-starred in the Disney comedy-drama film Cruella.