The former Llanelli Scarlets back-rower led the Exiles to promotion last season and is the club’s longest serving player.
He will lead the Premiership new-boys in his sixth full season and is relishing the new challenge.
“I love the club. I’ve been here so long that I feel part of the furniture,” said Mills.
“London Welsh is very special to me and I’ve got a lot of very happy memories here.
“I’ve made friends for life and everyone has always been great to my wife and I, while on the rugby side it’s great that we’ve taken the club to the Premiership.
“When I first arrived [January 2007] we were fighting relegation, but London Welsh has always shown an ambition to get promoted and personally it’s fantastic to have achieved that goal with the club.
“It’s exciting times; not knowing until so late hasn’t helped our preparations but the news definitely put a spring in our step coming in for the first day of pre-season on the Monday. There was real enthusiasm among the boys and now we’re going to give it our best shot.”
Mills, who has represented Wales at under-18, under-19 and under-21 level, came through the ranks at Bath before joining Llanelli.
He went on to make 38 appearances for the Scarlets and initially joined the Exiles on loan.
“I’m delighted for Jon; he’s definitely always had the potential to become a Premiership player,” added London Welsh head coach Lyn Jones.
“He had an intermittent season last year, but finished it strongly and was a big influence on London Welsh’s game.
“Next season is a great opportunity for him to fulfil and realise his potential, which he showed some years ago. I hope he can step up and show what he can do.”
