Millar Mills began his career with Yorkshire Carnegie, followed by a first spell at Edinburgh and three years with Ealing Trailfinders.
He moved into the Premiership by joining Wasps in 2021 but was left without a club when they went out of business the following year.
“His story is unique; after what happened at Wasps, he bounced around and played with the Barbarians, Bath and Edinburgh,” said director of rugby Phil Dowson.
“To then find a home with us, where he felt comfortable to be himself and felt comfortable to put those performances out on the pitch, shows his character.
“We’ll keep pushing him to achieve more and, as with everyone in our squad, I want to make sure we get the absolute best out of him.”