Thomas, 22, spent much of last season in Skolars colours, as they narrowly missed out on the play-offs.
The Kent-based forward started his rugby league career with West London Sharks, before joining the Super League club’s academy, making his first team debut for Harlequins against Bradford in 2011.
He said: “I really enjoyed my time at Skolars last season, and that’s one of the main reasons I decided to sign again this year. I’d played with most of the boys there when I was in the Academy and it was good to be back playing with them again.
“I had a few options but as I’m from London I wanted to stick with London and just see how far we can get with the Skolars. We did well last year and I think we can go one step further next season. I believe we can get into the top two and get promoted.”