Like fellow Irish actor Colin Farrell, he attended the Gaiety School of Acting in Dublin where he made his non-professional stage debut in the school's production of "Tear Up The Black Sail". His professional stage debut was in 1994's True Lines, directed by John Crowley, who recently directed Intermission. True Lines was first performed in Kilkenny however it then moved to the Dublin Theatre Festival and on to Bush Theatre in London.

He got his start in movies with roles in short Irish films such as Godsuit and Summertime but it wasn't until he landed a part in a 1996 UK / Ireland co-production called Trojan Eddie that he got into feature length film. In 1997 he took a lead role in the British film Shooting Fish and it was with this part that his career started to take off.

After his appearance as the outrageous seductor in the title role of About Adam he started to be noticed in the United States by critics and the public alike. However, in the summer of 2000 he briefly returned to the London stage in the Tennessee Williams play Orpheus Descending, starring himself as Val Xavier and Helen Mirren as Lady Torrance.

In recent years he has appeared in a number of big budget films such as Queen of the Damned, opposite the late Aaliyah, as the vampire Lestat de Lioncourt and The League of Extraordinary Gentlemen as Dorian Gray.

In 2005 Townsend starred alongside Gabrielle Union in Night Stalker, the 2005 remake of the 1974 ABC tv series Kolchak: The Night Stalker. He played the part of Carl Kolchak, an investigative reporter looking into his wife's murder. Night Stalker was abruptly cancelled by ABC after 6 episodes.

Townsend currently resides in Los Angeles with his long term girlfriend Charlize Theron, whom he met while filming Trapped in 2001.