Zagger's first professional acting role was portraying a young Marti Pellow in the Wet Wet Wet music video I Remember. Other roles soon followed; from 1990–1991, Zagger played Mike Bentley in the children's TV drama Grange Hill, and later in 1991 he played the role of "Razors" in the Granada Television series Up the Garden Path. He also had several guest roles in Minder, Casualty, Wycliffe, Dangerous Lady and The Bill.
In 1999, Zagger auditioned for the role of Dale Smith in The Bill, but the role went to his best friend Alex Walkinshaw and the character of PC Nick Klein was created for him. Zagger remained in the role for five years. Zagger left the series and took the role of Antipholus of Syracuse in the Stafford festival's production of Shakespeare's The Comedy of Errors.
In 2006, he appeared in the BBC series Robin Hood in the episode "A Thing or Two About Loyalty" as Lambert.
In July 2007, Zagger appeared as Herod in the second series of BBC/HBO TV series, Rome.
In January 2008, he made an appearance as Colt Winchester (Colin from Winchester) in the 5th episode of series three of MIHigh called "Dark Star".
In January 2010, he made an appearance as Farris Andreou in the second episode in the second series of Law and Order UK called "Hidden".
From November 2011 to April 2012, he starred in Comedy of Errors alongside Lenny Henry at the National Theatre in Southbank, London.
In June 2012, he made an appearance, alongside Luke Goss as Cesar in the feature film Interview with a Hitman, directed by Perry Bhandal.
In March 2013, he performed in the stage play Hope, written and directed by Scot Williams and co-starred with Samantha Womack and Mark Womack. The play ran at the Royal Court Theatre, Liverpool. In 2014, he joined the cast of the new musical Made in Dagenham, originating the role of Stan.
