Marnix Heersink, at a striking 6'4", started playing as a freshman on the men's basketball team in the 1966-67 season. During his first year, he was one of the top players on the UWO team with 172 points and the following year was the team's second-leading scorer. He was always praised for being an inspirational player who could bring the team up when they needed it most. Because of his high-scoring ways and inspirational nature, it was a huge loss when he injured his knee during the 1967-68 season and had to miss a few games to recuperate.
The Cage Mustangs were extremely worried that Heersink’s absence would have an impact on the teams success - which it actually did. He returned to playing in February 1968, and helped the team to get back on the right track. Staying for the rest of the season, Heersink and the Mustangs made it to the Ontario University Athletic Association tournament and won the championship title that year. Despite picking up a few too many fouls during the tournament, Heersink was voted the most valuable player of the tournament. For this, he received the Father E.C. LeBel Plaque – given to the most outstanding player in the league championships, which was awarded for the first time that year.
After this season, Heersink decided to take time off from the UWO basketball team.The knee injury of the previous year played a part in his decision, but it was also because he wanted to focus on his medical school ambitions. He returned to play once more in the 1971-72 season, and that year was named as a first-team All-Canadian player. Even though his career with the team was plagued by his injury, he was an inspirational player who had a great impact on them. After this, he went on to pursue his dream of a medical career.