Leen got his first taste of professional hockey at the end of the 2014-15 season, when he joined the Bakersfield Condors, after completing four years of college hockey at the University of Maine. The 23-year-old made his pro debut with the Condors on March 13, and went on to appear in 11 games, tallying six points (3g-3a). Two of those goals came on April 10 against Stockton.
While attending the University of Maine, Leen recorded 53 points (24g-29a) and added 79 penalty minutes in 120 career games. The Chester, N.J. native had his most productive season as a junior in 2013-14, when he picked up 23 points (6g-17a), while also receiving the Richard Britt Perseverance Award, before netting a career high in goals (10) as a senior.