The 19-year old right-back has been with the Jambos since he was a kid but will not be offered a new contract after suffering injuries recently.
Mullen, who was forced out of the Scottish Communities League Cup final against St Mirren because of a broken foot, posted the following on Twitter.
Not being offered a contract for next season at hearts. Enjoyed being at the club since I was 12 but annoyed the way last few months ended
with the injury. Absolute gutted at leaving some of the boys gona miss them a lot