Macken was studying for a Bachelor of Science degree in Psychology at University College Dublin when he began his modelling and acting career at the university’s fashion show in 2002 and 2003, after which he joined the model agency Morgan the Agency and was selected as the face of the Abercrombie and Fitch 2003 campaign. He modelled for Ralph Lauren and GQ, and was the face of the 2008 Braun campaign.

Macken’s first major film role was in Studs in 2005, which starred Brendan Gleeson and Emmett J. Scanlan. He went on to play leading roles in indie films Triple Bill and Rise of the Bricks. After the death of his father in 2007, Macken took a break from acting. He returned to performing the following year in the role of drug dealer Gavin Cluxton in Fair City. He played the part for more than four months and also appeared live on the show Fair City Sings, where he performed the song “Hallelujah”.

Macken wrote and edited his debut feature film Christian Blake, which premiered at the Mid Ulster Film Festival 2008, before playing in the 2008 Galway Film Fleadh. It was released on 19 May 2009 with Celebrity Distribution and the sales company Around The Scenes Inc. He co-wrote and directed the song and video for Una Healy’s song “Don’t Leave Me Alone”. It was the title track for the no-budget indie film, for which Healy wrote much of the soundtrack.

In 2008, he wrote, directed, and acted in his second feature film, Dreaming For You. The film featured a soundtrack composed by Dublin band, The Evora. The film was selected for the 2007 Los Angeles Short Film Festival, and for the 2009 Galway Film Fleadh.

In 2009–10, Macken had roles in the BBC drama Small Island (playing an American soldier named Calhoon), in Neil Marshall’s film Centurion, and in the second series of RTÉ’s Raw. He appeared in Storyland, as well as Lee Cronin’s film Through The Night, opposite Nora Jane Noone, and Siren opposite Anna Skellern. He directed an hour-length documentary on the Irish fashion industry The Fashion of Modelling which screened on RTÉ 2 on 24 May 2009.

Between 2010 and 2012, Macken played the recurring character of Gwaine (loosely based on the image of Gawain) in the BBC One TV series Merlin. He appeared in four episodes of the third series and all episodes of the fourth and fifth.

In October 2012 Macken completed shooting for Cold, a feature film released in 2013 written and directed by Macken, and he stars in the film alongside former co-star, Tom Hopper. The plot centres around two disconnected English brothers who are outcasts in a small village in the west of Ireland. Drawn back together by the unexpected death of their father, they are immediately at odds with each other until they find a girl dumped in the moors. The film was partially crowdfunded through an Indiegogo campaign. The film was later released under the name Leopard.

After directing a number of independent features, in 2018 Macken adapted Rob Doyle’s award-winning novel Here Are the Young Men into a screenplay, and directed the resulting film.