Magennis hasn’t played since the Magic Weekend defeat to Wigan last May, and a troublesome knee injury has forced him to call time on his professional rugby league career.
He said: “My knee has been bad for a few years now, but not just last season; since I was 17 really. It was getting worse and worse and I kept having surgery on it.
“It came to a point last year when I couldn’t train fully and I stopped looking forward to training as my knee was that sore.
“When I did train it was swelling up and I wasn’t fully fit. I spoke to my surgeon and he said he could do an operation so I could get on with the rest of my life. In the end I chose that and now I have to move on.
“I haven’t done what I wanted to do like win trophies and play for GB, but that is life. I have played rugby since I was 7 and have put a lot into it. Now I have to do that in another career.
“I’d like to thank the fans for their support and the coaches I have worked with. Eric Chisnall, Derek Traynor and Mike Rush all put a lot of faith in me. I’d also like to thank my family for being there.”
He previously spent 19 months on the sidelines with a serious leg injury before forcing his way in to the Saints first team in 2010, making his debut against Hull.
The former Blackbrook amateur has scored four tries in 33 appearances for Saints, and was given the number 18 in Nathan Brown’s 2013 squad list last month.