Stewart, 30, has played 184 NRL matches for the Club since making his debut against the Brisbane Broncos back in Round 3, 2003. He has played in four NRL Grand Finals (2007, 2008, 2011 and 2013), winning premierships in 2008 and 2011 as well as the Clive Churchill Medal in 2011.
Sea Eagles General Manager David Perry has acknowledged it is disappointing that the club was unable to retain Glenn and thanked the premiership winning forward for his service to the club.
"Glenn has not only been a quality player but one of the influential leaders that has driven recent success for the club over a significant period of time," Perry said.
Sea Eagles Coach Geoff Toovey also paid tribute to the service Glenn has provided the club with since he debuted back in 2003 against the Brisbane Broncos.
"Glenn has been instrumental to the successes of the Club for more than a decade and I'm glad that he now has comfort in knowing his future is secured."
"It's been an emotional time for many at the club, but I hope this can help ease the pressure on Glenn and now that his future is secured, he can focus back on enjoying playing in the NRL with his mates."
Stewart said he was disappointed to be leaving the club he has called home for 12 seasons.
"It's unfortunate that circumstances have forced me to leave the Club that means so much to me, but I've loved my time at Manly and will be giving my all to the very last minute - that can be assured," Stewart said.
Stewart also took time to thank the members and fans for their phenomenal support over all the years.
"The support everyone has given me over the years has been immense," he said.
"It's going to be an odd feeling when I do have to come back to Brookie next season in a different jersey, but I will never forget the support the members, fans and supporters have given me over the years."