MacDonald grew up in Brighton, Michigan, where he played in Compuware's midget hockey program before progressing to junior hockey in the United States Hockey League with the Waterloo Black Hawks. He committed to and played collegiate hockey at Cornell University of the Hockey East. While developing his defensive awareness with Cornell, MacDonald struggled offensively at the collegiate level, registering just 21 points through 104 games in his four-year career. As a free agent following his senior season with the Big Red in 2014–15, MacDonald made his professional debut in signing with the Elmira Jackals of the ECHL to end the regular season on March 26, 2015. In continuing with the Jackals in the 2015–16 season, MacDonald rediscovered his offensive touch in recording 17 goals and 37 points from the blueline to be selected to the ECHL's All-Rookie Team. He made his debut in the American Hockey League appearing in a one-game on loan with the Springfield Falcons. In the 2016–17 season, MacDonald returned to the ECHL with the Toledo Walleye before signing an initial try-out contract with the Albany Devils, the New Jersey Devilsfarm team, on January 5, 2017. In making an early impression with Albany he soon secured an AHL deal for the remainder of the season and an extension after he scored eight goals and 24 points in 34 games to close out the year. In his first full season in the AHL in 2017–18, MacDonald led the league's defensemen with 20 goals and 55 points and earned an AHL All-Star berth with the Binghamton Devils. Despite missing the post-season, he was selected to the AHL's First All-Star Team. As a free agent, MacDonald opted to leave the Devils organization and sign his first NHL contract in agreeing to a two-year, two-way contract with the Florida Panthers on July 3, 2018. After attending the Panthers 2018 training camp, MacDonald impressed to make Florida's roster for the 2018–19 season. He made his NHL debut on opening night, and scored on just his third shift, and first career shot in a 2–1 shootout defeat to the Tampa Bay Lightning on October 6, 2018. After two games with the Panthers MacDonald was re-assigned to AHL affiliate, the Springfield Thunderbirds, for the remainder of the season. MacDonald assumed a top-pairing role with the Thunderbirds and in 72 games he led all blueliners in scoring with 14 goals and 43 points. Following his first season in Florida, on June 29, 2019, MacDonald was traded by the Panthers to the Colorado Avalanche in exchange for Dominic Toninato.
