Film:
* The Theory of Everything (2014): Played Jonathan Jones, Stephen Hawking's former research assistant.
* The King's Speech (2010): Played Lionel Logue's brother, a supporting role.
* The Lost City of Z (2016): Played the role of William Fawcett.
* The Woman in Black (2012): Played Arthur Kipps, the protagonist.
* The Last Legion (2007): Played the role of Romulus Augustulus.
* Bright Star (2009): Played Charles Wentworth, a supporting role.
Television:
* The Crown (2016-present): Plays Lord Mountbatten, recurring role.
* The Last Kingdom (2015-2022): Played the role of Father Beocca, a main character.
* The Shadow Line (2011): Played the role of Detective Superintendent Joseph "Joe" Caldwell, a main character.
* The Inspector Lynley Mysteries (2001-2008): Played the role of Detective Sergeant Havers, a main character.
* Spooks (2002-2011): Played the role of Tom, a main character in season 3.
* The Lost World (2001): Played the role of Lord Roxton.
* The Bill (1984-2010): Played the role of D.I. "Danny" Beck in 1999.
* The Vicar of Dibley (1994-2007): Played a supporting role.
This is not an exhaustive list, as Alastair Mackenzie has been in many other projects throughout his career. You can find a complete list of his work on his IMDb page.