Arnold committed to play college hockey at Boston College, where he joined the Eagles program in 2010. As a freshman in 2010–11, he recorded 20 points in 39 games. In his sophomore season, he joined the US National Junior Team for the 2012 World Junior Championship. Arnold recorded three goals and three assists in six games for the seventh place Americans. En route to a 36-point season with the Eagles, Arnold scored the winning goal in overtime to defeat Boston College's rival Boston University Terriers and win the 60th annual Beanpot tournament. He then capped his season with a national championship as Boston College won its final 19 games, culminating in a 4–1 victory over the Ferris State Bulldogs to win the 2012 Frozen Four tournament.