The 45-year-old Hull native has inked a two-year deal at the club, and will work as one of a three-man team of assistant coaches.
“Steve has tremendous coaching credentials through his time as England’s head coach as well as his experience with Bradford in the Super League and now the Roosters in the NRL,” said Warriors managing director Jim Doyle.
“In joining Stephen, Cappy (Andrew McFadden) and Stacey (Jones), Steve completes our NRL coaching staff for next season.
“We’re excited about being able to bring this group of coaches together as we strive to lift our standards.
“Certainly our senior players and the whole NRL squad will now have better coaching support than they’ve ever had before.”
McNamara is ready to build on his previous experience to help the Warriors improve.
“Now I’m really excited about this next step in my coaching career, coming to Auckland and working with Stephen and a great coaching group.
“I am in no doubt that the Vodafone Warriors have the players to make a mark in the NRL and I want to do all I can to help make that happen.
“I have the greatest admiration for Stephen from the days when we were opposing coaches of our national teams. He has done wonderfully well with the Kiwis and I’m sure he’s going to do a terrific job with the Warriors.”
Warriors head coach Stephen Kearney is delighted that McNamara has joined the club.
“While Steve and I were adversaries in our roles with our national teams we formed a great relationship,” he said.
“He brings qualities that will give us a really strong and experienced coaching group as we look ahead to take the Warriors to the level we collectively expect of this club.
“There’s much work to do but with Steve, Cappy and Stacey we have a coaching team that can really lift standards and bring about the improvements we must make to be consistently competitive in the NRL.”