McDowell was born Malcolm John Taylor in Horsforth, then in the West Riding of Yorkshire, the son of Edna and Charles Taylor. However, six weeks after McDowell was born, his family relocated to the east coast of Yorkshire because his father was in the Royal Air Force.
McDowell began his professional life serving drinks in his parents' pub and then as a coffee salesman. While enrolled in Cannock House School, he began taking acting classes, and eventually secured work as an extra with the Royal Shakespeare Company. McDowell made his screen debut as school rebel Mick Travis in If.... (1968) by British director Lindsay Anderson.
McDowell mainly portrayed antagonists in the late 1970s and 1980s, including the title character in the Caligula (1979).
McDowell played himself in Robert Altman's The Player. In the 2003 film, I'll Sleep When I'm Dead.
In 2006, McDowell portrayed radio mogul Jonas Slaughter on Law & Order: Criminal Intent, In 2007, he portrayed the wealthy, conspiratorial villain Mr. Linderman during the first season of the NBC hit series Heroes, a role he reprised in the third season premiere. He has starred in Jerry Was a Man, which has appeared as an episode of Masters of Science Fiction, on ABC and Sky. He also portrayed Terrence McQuewick on Entourage and Julian Hodge on Monk.
McDowell appeared in Rob Zombie's 2007 remake of the horror film Halloween, and its sequel Halloween II.In December of 2009 McDowell made an appearance in the music video "Snuff" by the band Slipknot.
McDowell was married to actress Margot Bennett from 1975 to 1980. He then married actress Mary Steenburgen, whom he had first met while filming Time After Time and they had two children together: Lily Amanda (born 21 January 1981), and Charles Malcolm (born 10 July 1983), before divorcing in 1990. In 1991, McDowell married Kelley Kuhr, with whom he has three children: Beckett Taylor McDowell (born 29 January 2004), Finnian Anderson McDowell (born 23 December 2006) and Seamus Hudson McDowell (born 7 January 2009). He currently resides in Ojai, California. He is the maternal uncle of actor Alexander Siddig, who appeared in Star Trek: Deep Space Nine, Kingdom of Heaven, Syriana and 24. He and his nephew both appeared in the movie Doomsday by director Neil Marshall.