Mostert (5-10, 197) originally signed with the Philadelphia Eagles as an undrafted free agent on May 2, 2015. After signing with Philadelphia's practice squad on Sept. 6, 2015, he was signed to the Miami Dolphins active roster on Sept. 15, 2015. Mostert went on to appear in one game with Miami, seven games with the Baltimore Ravens and three games with the Cleveland Browns in 2015, where he returned a total of 19 kickoffs for 530 yards (27.9 average). This season, Mostert was waived by Cleveland on Sept. 4, 2016, signed to the New York Jets practice squad on Sept. 6 and later released from the Jets on Sept. 12. He was signed to the Chicago Bears practice squad on Sept. 13 where he spent time between the practice squad and active roster, appearing in two games with Chicago prior to being waived on October 21.