Leonard was originally first eligible to be drafted in 2016, but was not selected while playing for the Green Bay Gamblers of the United States Hockey League. After two years with the Gamblers, Leonard joined the University of Massachusetts Amherst ("UMass"), playing for their ice hockey team, the UMass Minutemen while studying Communications and Sport Management. While with the Minutemen, Leonard caught the attention of the San Jose Sharks, who drafted him in the sixth round, 182nd overall in the 2018 NHL Entry Draft despite not having any contact with Leonard during the predraft process. In his third year at UMass, Leonard was a finalist for the Hobey Baker Award and lead all NCAA players in goals, with 27.