[2] His early acting appearances include TV series such as Mrs Thursday (1966), The Power Game (1966) and Market in Honey Lane (1967), and a small role in the film Zeppelin (1971).
After playing the lead role in an episode of the ITV cult anthology series, Hammer House of Horror in 1980, Lonnen was given his next lead role in 1982 in the ITV mini-series Harry's Game, based on the novel by Gerald Seymour, playing Harry Brown, a British agent sent to Northern Ireland to smoke out the IRA assassin of a cabinet minister.
He also appeared in the 1985 thriller movie Murder Elite, opposite Ali MacGraw, Billie Whitelaw and Hywel Bennett.
He appeared in several episodes of the Canadian sci-fi series Starhunter and had a recurring role in the short-lived revival of the ITV soap opera Crossroads.
His stage credits include Under Milk Wood (as the Narrator), Same Time, Next Year, In Praise of Love, Run for Your Wife, Wonderful Town, Lock Up Your Daughters, Guys and Dolls, Bells Are Ringing, Rebecca and The Perfect Murder.