Allen's first professional appearance was in a one-off
Channel 4 comedy,
You Are Here in 1998, co-written by
Matt Lucas and
David Walliams. The same year, Allen and his sister Lily appeared in the 1998 film
Elizabeth, which was produced by their mother. Starting in
Chichester on 31 January 2008, he took over
Daniel Radcliffe's role in a revival of
Equus on a nationwide tour. In April 2009, Allen co-starred with then-partner
Jaime Winstone in the music video for "
Dust Devil" by
Madness. He also had a role in the
BBC2 film,
Freefall. He continued to work in films, appearing in
Freestyle,
Soulboy, and
The Kid in 2010. Allen came to international attention when he was cast as
Theon Greyjoy in the HBO fantasy series
Game of Thrones, an adaptation of
George R. R. Martin's
A Song of Ice and Fire, in 2011. he appeared as a series regular for 8 seasons and was nominated for a
Primetime Emmy Award for Best Supporting Actor in 2019 for the show's final season. Allen starred in the 2012 British thriller
Confine, and then opposite
Keanu Reeves in the 2014 film
John Wick. In 2016, directed by
Mike Christie, he filmed a two part documentary for the
History Channel titled
Football: A Brief History, about the roots of association football. That same year, he played Ringwood in the
BBC Two 1940s-set miniseries
Close to the Enemy and joined the London production of
Jesse Eisenberg's
The Spoils at
Trafalgar Studios. Allen co-starred in the sci-fi sequel
The Predator as Lynch in 2018, and
Taika Waititi's
Oscar-nominated dark comedy
Jojo Rabbit as Finkel as well as the coming-of-age film
How to Build a Girl as John Kite in 2019. He had a recurring role as Isaac Pincher in the third series of the period drama
Harlots. In 2020, Allen appeared in the
ITV crime drama
White House Farm as Brett Collins. Allen made his
Broadway debut in 2022 when he took over the role of Mooney from
Dan Stevens in
Martin McDonagh's
Hangmen at
John Golden Theatre. For his performance, Allen received a
Tony Award nomination for
Best Featured Actor in a Play. In 2022, he starred as Lieutenant
Jock Lewes in the
BBC One World War II miniseries
SAS: Rogue Heroes. ==Personal life==