Gallagher played the lead role of compassionate consultant Ewart Plimmer in the first three years of BBC TV's long-running medical drama series Casualty. In ITV's daytime legal drama series Crown Court, he played Barrister Jonathan Fry.
Gallagher's numerous other credits include Heartbeat, The Sweeney, Bergerac, Bad Girls, Wycliffe, Midsomer Murders and London's Burning. In a rare comedic role, he played DS Lang in the "Photographs" episode of Some Mothers Do 'Ave 'Em.
Gallagher also portrayed Bill Molesley in the series Downton Abbey.