Maskell's first television appearance was in 1991 in the ITV police drama The Bill. Between 1992 and 2002, he made a further five appearances on the show, portraying a different character each time. He has had one-off roles in a number of other popular British serial dramas such as Casualty, London's Burning, Soldier Soldier. His most regular television roles have been in the 2005 comedy series Mike Bassett: Manager, in the comedy Sketch show The Wrong Door, and in the 2013–2014 thriller drama action series Utopia. Since 2015, he has been a series regular in sci-fi thriller Humans.
Maskell was born in London. As a youngster, he played football for Long Lane JFC as a full back. He first trained in acting at the Anna Scher Theatre in Islington, London, where he attended classes from the age of 11, and studied at the Miskin Theatre, Dartford at North West Kent College from 1992. He later worked as a Director at the Miskin Theatre.
Maskell's film career began in 1997 with an appearance as Schmuddie in Gary Oldman's directorial debut Nil by Mouth. His leading role in the 2011 thriller Kill List attracted positive reviews from critics and saw him nominated for the Best Actor award at the British Independent Film Awards.
