The midfielder returns to Scotland from English League One side Barnsley on a four-year contract.
The 22-year-old moved from Paisley in the summer of 2017 having made over 100 appearances for the Buddies.
Described by Hibs as an “dynamic attacking midfielder”, Mallan couldn’t force his way into the first-team at Oakwell but will now look to follow in the footsteps of former St Mirren team-mate John McGinn by starring at Easter Road.
Mallan said: “I am delighted to have signed for Hibs. It’s an exciting move for me – the club obviously had a good season last year and I want to be a part of that moving forward.
“I didn’t get much game time last year and I want to be back playing. The recruitment team at Hibs and the gaffer made me feel really wanted, which made me feel that this was the ideal place for me.
“It’ll be hard joining a team that’s been doing well but I’m looking forward to getting started and trying to get a place in the side and it’s a challenge that I’ll relish.
“I always enjoyed playing at Easter Road, it’s a great stadium when full and I’m sure it will be even better playing for the home side there.
“I’m just really pleased to have put pen to paper on the deal and I can’t wait to get started.”