Arnold committed to play college hockey at Boston College, where he joined the Eagles program in 2010.[1] As a freshman in 2010–11, he recorded 20 points in 39 games.[4] 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,[4] Arnold scored the winning goal in overtime to defeat Boston College's rival Boston University Terriers and win the 60th annual Beanpot tournament.[5] 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.[6]
Following a 35-point season in 2012–13, Arnold completed his senior season on a line with close friends Johnny Gaudreau and Kevin Hayes. Arnold was praised by both his linemates and coach, Jerry York. They lauded him as being a player who excelled in all three zones of the ice, and who sacrificed personal stats for the benefit of his linemates.[4] Arnold finished his college season with 14 goals and 53 points. For his college career, he recorded 159 points in 144 games.[7]
Professional
Following Boston College's 5–4 loss in the 2014 Frozen Four semifinal to the Union Dutchmen, Arnold joined Gaudreau – who was also a Flames draft pick – in signing their first professional contracts.[8] Arnold signed a two-year contract worth an average of $900,000 per season.[9] He made his NHL debut in the Flames final game of the season, a 5–1 loss to the Vancouver Canucks on April 13, 2014.[10] Arnold thus earned him a spot on the list of players who played only one game in the NHL.
Arnold spent the entirety of the 2014–15 and 2015–16 seasons playing for Calgary's AHL affiliate. He produced 24 goals and 60 points in 113 games over both seasons.