The 19-year-old, who left Tynecastle earlier this summer before the Gorgie side entered administration, is Pat Fenlon's fourth signing of the summer.
"A former Scotland under-17 internationalist, right-back Mullen, 19, agreed the deal on Saturday morning and has become the club's fourth arrival during the summer transfer window."
Hibs will depart for a training camp in southern Europe on Monday afternoon following their 1-1 draw with Dumbarton in their first pre-season friendly on Saturday.
- Sports & Recreation
- Fraser Mullen
- Owain Tudur Jones
- Pat Fenlon
- Liam Craig