Mullin scored just five goals last season but became a Wrexham legend across four years with the club, scoring 105 goals in 165 appearances as they climbed from non-league into the Championship. The club's co-owner, Rob McElhenney, paid tribute to the striker on his departure.
"It's only a short career and as much as I love Wrexham, I want to play football," Mullin added on why he decided now was the time to move on.
"I don't want to be sat at home every weekend.
People say it's the dream to be a footballer, but it's so difficult when you're training every day and at the end of the week, you don't get to do what you trained for. It's really tough."