He has previous played for the Chicago Blackhawks, Vancouver Canucks and St. Louis Blues Highmore was drafted eighth overall by the Saint John Sea Dogs of the Quebec Major Junior Hockey League (QMJHL) in the 2012 QMJHL entry draft.

[2] Despite his success as an alternate captain in the QMJHL, Highmore was passed over at the NHL entry draft twice, the second time being in his last year of eligibility.

[3][4] In his last year with the Sea Dogs, he was the only player who earned a professional hockey contract without being drafted.

[6] Highmore started the 2017–18 season with the Blackhawks American Hockey League (AHL) affiliate, the Rockford IceHogs.

[7] After leading his team in points, Highmore was selected to participate in the 2018 AHL All-Star Classic along with teammate Carl Dahlström.

[18] After rehabilitating, Highmore spent the summer training in Halifax with other NHL and AHL players.

[20] He was subsequently named an alternate captain alongside Jacob Nilsson and Tyler Sikura.

In January 2023, on the basis of his excellent play with Springfield, Highmore was named to the 2023 AHL All-Star Game.

[33] As a free agent, Highmore was signed to a one-year, two-way contract with the Ottawa Senators on July 1, 2023.

He was placed on waivers on October 2, 2023[35] and after going unclaimed, was assigned to Ottawa's AHL affiliate, the Belleville Senators.