Hartman joined the Plymouth Whalers of the Ontario Hockey League for the 2012–13 OHL season and was invited to take part in the CHL Top Prospects Game. He was then selected to play with the gold-medal-winning American squad at the IIHF World U20 Championship. Hartman was rated as a top prospect, fulfilling the expectation to be a first-round selection at the 2013 NHL Entry Draft. On November 18, 2013, the Chicago Blackhawks signed Hartman to a three-year entry-level contract. Hartman made his NHL debut on February 13, 2015, against the New Jersey Devils. Hartman started the 2016–17 NHL season with the Blackhawks. The team sought to harness Hartman's physical play style after trading away Andrew Shaw in the offseason. He scored the first goal of his career against Jake Allen of the St. Louis Blues during the NHL's season opener on October 12, 2016. Hartman scored his first career hat trick against the Nashville Predators on January 8, 2017. Hartman finished the season with 19 goals and 12 assists. During the 2017–18 season, Hartman recorded eight goals and 17 assists in 57 appearances for the Blackhawks. He rotated through the Blackhawks lineup, but primarily skated as a bottom-six forward. On February 26, 2018, Hartman along with a fifth-round pick in 2018 NHL Entry Draft was traded to the Nashville Predators in exchange for Victor Ejdsell, first-round and fourth-round picks in 2018. He appeared in 21 regular season games for Nashville, where he tallied three goals and three assists. The NHL suspended Hartman for one game during the first round of the 2018 Stanley Cup playoffs after he delivered an illegal check to Carl Soderberg. In the 2018–19 season, at the NHL trade deadline on February 25, 2019, the Predators traded Hartman, along with a conditional fourth-round pick in 2020, to the Philadelphia Flyers in exchange for Wayne Simmonds. He added two goals and six points through 19 games with the Flyers as the club missed the playoffs. On June 24, Hartman was again involved in a trade, dealt by the Flyers to the Dallas Stars in exchange for Tyler Pitlick. The following day, Hartman was not tendered a qualifying offer from the Stars, enabling him to become a free agent on July 1. On the opening day of free agency, Hartman was signed to a two-year $3.8 million contract with the Minnesota Wild on July 1, 2019.
International play
Hartman competed for the United States in the 2012 IIHF World U18 Championships. He recorded six points, two goals and four assists, in six games. As a member of the American team, Hartman received a gold medal. Hartman played for the U.S. in the 2013 World Junior Ice Hockey Championships. He accumulated three points, two goals and an assist, in seven games. As a member of the U.S. team, Hartman received a gold medal. He posted four points with the U.S. team in the 2014 World Junior Ice Hockey Championships, two goals and two assists, in five games.
Personal life
Hartman originally committed to playing college hockey at Miami University, but later de-committed from the school to play for the Plymouth Whalers. His younger brother Tanner plays hockey for the Chicago Fury U15 team. He has a white German Shepherd named Riley. Hartman also played football for the Bloomingdale Bears travel football team along with his friend and former teammate Vinnie Hinostroza.
