Mortimer has joined the Tricolours on a two-year deal after securing a release from the Eels.
Son of former Canterbury great Peter Mortimer, the 22-year-old five-eighth enjoyed a successful 2009 season, helping the Eels reach the Grand Final.
The same year he was also a Dally M Rookie of the Year finalist but has failed to make an impression since the arrival of coach Stephen Kearney.
Roosters recruitment manager Peter O’Sullivan believes Mortimer will be a valuable asset to the club.
“Daniel suits the Roosters style of play and with his ability to play five eighth or halfback, we see him playing a key utility role, adding great depth to our halves alongside the likes of Braith Anasta and Mitchell Pearce,” O’Sullivan said.
“Daniel is widely regarded as a quality person on and off the footy field and we are looking forward to watching him develop as part of our team over the coming seasons.”
Mortimer will join the Roosters for pre-season training next month.