2009-10: Joey Diamond skated in 32 games for the University of Maine as a freshman. He scored 9 goals with 3 assists and was -7 with 73 penalty minutes. Maine finished tied for fourth in Hockey East. The Black Bears lost to eventual national champion Boston College in overtime in the Hockey East championship game but did not receive a bid to the NCAA tournament.
2010-11: Diamond played 33 games for Maine in his sophomore season. He led the Black Bears in penalty minutes (130) while scoring 11 goals with 10 assists and finishing +8. Maine finished fifth in Hockey East and lost to Merrimack in the quarterfinals of the league tournament.
2011-12: Diamond led the University of Maine in goals (25) and penalty minutes (117) as a junior. He also had 22 assists and was +10 in 37 games for the Black Bears. Maine finished fourth in Hockey East and reached the conference championship game; falling to Boston College for the second time in three years. Maine reached the NCAA tournament for the first time since 2006-07 and lost to Minnesota-Duluth in the Northeast Regional.
2012-13: Diamond made his pro hockey debut with the AHL's Bridgeport Sound Tigers in March following his senior season at Maine. He signed a one-year NHL contract with the Islanders, Bridgeport's parent club, in April 2013. In ten games with Bridgeport he scored 2 goals with 3 assists and was -2 with 27 penalty minutes. The Sound Tigers missed the AHL playoffs after finishing third in the Northeast Division. Diamond was a captain for Maine, once again leading the Black Bears with 14 goals and 147 penalty minutes, and was +6 with 10 assists. Maine finished eighth in Hockey East and was eliminated by eventual Frozen Four participant Massachusetts-Lowell in the Hockey East quarterfinals.
Diamond signed an entry-level contract with the New York Islanders in April of 2013.