Chicago Blackhawks

On November 18, 2013, the Chicago Blackhawks signed Hartman to a three-year entry-level contract.^[12] Hartman made his NHL debut on February 13, 2015, against the New Jersey Devils.^[13] 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.^[14] 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.^[15] Hartman scored his first career hat trick against the Nashville Predators on January 8, 2017.^[16] Hartman finished the season with 19 goals and 12 assists.^[17]

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.^[18]

Nashville Predators

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.^[19] He appeared in 21 regular season games for Nashville, where he tallied three goals and three assists.^[20] 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.^[21]

Philadelphia Flyers

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.^[22] He added two goals and six points through 19 games with the Flyers as the club missed the playoffs.

Minnesota Wild

On June 24, 2019, Hartman was again involved in a trade, dealt by the Flyers to the Dallas Stars in exchange for Tyler Pitlick.^[23] The following day, Hartman was not tendered a qualifying offer from the Stars, enabling him to become a free agent on July 1.^[24]

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.^[25]

On April 22, 2021, Hartman signed a three-year, $5.1 million contract extension with the Wild.^[26]

On October 7, 2023, Hartman signed a three-year, $12 million contract extension with the Wild.^[27]

Hartman sparked controversy in late December 2023 after being accused of intentionally attempting to injure a player in a back-to-back series against the Winnipeg Jets.^[28] On December 30, Wild star Kirill Kaprizov was injured from a pair of cross-checks from Jets defenceman Brenden Dillon.^[29] The next night, 1:51 into the first period of a rematch against the Jets, Jets forward Cole Perfetti claims that Hartman told Perfetti that he was going to intentionally high-stick him as retribution for Kaprizov's injury, despite Perfetti not being involved in the play which injured Kaprivoz.^[29] On the ensuing faceoff, Hartman hit Perfetti in the face with a high-stick.^[29] Although the play was not called a penalty by the referees, Perfertti was wearing a microphone when the exchange occurred which picked up audio of the interaction.^[28] Perfetti was not injured on the play but required stiches from the high-stick.^[30] On January 2, 2024, the NHL fined Hartman $4,427.08 for high-sticking Perfetti, the maximum allowable under the league's collective bargaining agreement for an infraction which does not involve an injury or game misconduct penalty resulting from a play.^[30][31] After three days of silence following the breaking of the story, Hartman publicly denied that he had intentionally high-sticked Perfetti and instead claimed that Perfetti had baited Hartman into claiming responsibility for the incident during the match; audio of the exchange was not released after the Wild organization and National Hockey League Players' Association objected to its release.^[30][32]