A native of Needham, Massachusetts, Arnold attended and played high school hockey at Noble and Greenough School and the Boston Junior Bruins program before joining the USA Hockey National Team Development Program for the 2009–10 season. At the U-17 level, Arnold was team captain of a select team that participated in a five nation tournament in Slovakia; he was named the tournament's most valuable player. At the 2010 IIHF World U18 Championships, Arnold recorded three points in six games as National Team Development Program, representing the United States, won the gold medal. At the 2010 National Hockey League (NHL) Entry Draft, Arnold was selected in the fourth round, 108th overall, by the Calgary Flames.