In June 2021, she qualified to represent Canada at the 2020 Summer Olympics in Tokyo. Mac Neil first competed as part of the Canadian team for the 4×100 m freestyle relay, replacing Ruck for the event final and swimming a 53.47 second split to help take the silver medal, Mac Neil's first Olympic medal. The following day, Mac Neil competed in the final of the 100 m butterfly event, taking the gold medal by a margin of 0.05 seconds over China's Zhang Yufei, setting a new personal best and Americas record of 55.59. She was the first Canadian gold medalist of the 2020 Tokyo Games. Mac Neil wears glasses and does not have contacts or prescription goggles, therefore she cannot immediately see her results and it took her a few seconds to focus on the results board to realize she won gold. Cameras focused on her squinting at the results board and she said after that "I was just trying to squint and see where I came. I heard my name getting called so I knew I must have done something good." Mac Neil's final event was the 4×100 m medley relay, where she posted a 55.27 time in her leg of the relay and the Canadian team won the bronze medal, Mac Neil's third of the Olympics.