The 49-year-old was head coach of the Titans for the 2018 season, as well as the first half of the 2019 season before being terminated by the club with just four wins on the board.
It was a miserable tenure for Brennan at the club, where he won just 12 out of 40 total games.
He will join the Knights as the head of junior pathways, and his past working with young talent will provide valuable to a Knights organisation desperately searching for the missing link.
He has coached at Jersey Flegg and Harold Matthews Level, before taking the Knights' under-20s team to the finals in 2011.
Brennan brings bid advantages to the Knights system, the place where he started his coaching career.
His role will encompass everything from grassroots right through to the NRL and NRLW teams, and Knights' CEO Phillip Gardner said it was a crucial milestone for the club.
“Garth has a proven record in the junior development space, having played a key role in the careers of some of the best players in our game,” Gardner said.
“Garth will be a tremendous asset for the Club moving forward and we can’t wait to have him join the team.”
“I can’t wait to get involved and I’m really looking forward to the challenge this role presents,” Brennan said.
“The Club at the moment is filled with quality people, including Adam O’Brien, Danny Buderus, Alex McKinnon and Head of Performance Hayden Knowles. The chance to work alongside them, as well as the Wests Group, is one I just couldn’t refuse.
“For my entire career I’ve been really passionate about strengthening junior development and pathways programs, which has been some of the most enjoyable and rewarding work of my life so far."