Chosen in the 7th round of the 2008 Entry Draft (188th overall) by the Toronto Maple Leafs, he would make his NHL debut on March 11, 2015, recording 1 point in a 4–3 Toronto victory over the Buffalo Sabres. He was also named 3rd star of the game and went on to finish the season with the Toronto Maple Leafs, playing a total of 12 games.[1] MacWilliam wore number 57 during his time with the Maple Leafs, becoming the first player in franchise history to wear the number.
On July 3, 2015, MacWilliam signed as a free agent to a one-year, two-way contract with the Winnipeg Jets.[2] On July 1, 2016, as a free agent for a second consecutive season, MacWilliam left the Winnipeg Jets to sign a one-year, two-way deal with the New Jersey Devils.[3]
On July 4, 2017, having left the Devils as a free agent, MacWilliam with slim NHL interest opted to continue in the AHL, signing a contract with the Rochester Americans, affiliate to the Buffalo Sabres.[4]
With the following 2020–21 North American season delayed due to the COVID-19 pandemic, MacWilliam opted to pursue a career abroad, agreeing to an initial try-out with German three-time champions, EHC München of the DEL. On December 2, 2020, MacWilliam agreed to remain in Munich for the remainder of the 2020–21 season.[5]
After two seasons in the DEL, MacWilliam left Germany and joined the neighbouring ICEHL, by signing a contract with the Red Bull sponsorship of EC Red Bull Salzburg on July 18, 2022.[6]
After a lone season with Salzburg, MacWilliam returned to former club, EHC München of the DEL, for the 2023–24 season on June 7, 2023.[7]
Before his professional hockey career, MacWilliam attended the University of North Dakota, where he also played hockey and was honoured as captain in his senior year. He graduated with a degree in Business Management.[9]