While playing juniors in Ontario, MacMillan skipped her team of Sarah Daviau, Lindsay Dubue and Marcia Richardson to a gold medal at the 2015 Optimist Under 18 Women's Championship.
[6] For her final two years of junior eligibility, MacMillan moved to British Columbia and joined the Sarah Daniels' Delta based rink.
The team, including second Megan Daniels and lead Sarah Loken had a successful tour season, reaching the semifinals of the Royal LePage Women's Fall Classic and the quarterfinals of the Driving Force Abbotsford Cashspiel.
The team entered the BC provincial junior championship as one of the top seeds and finished first after the round robin with a 6–1 record.
With a 2–2 record through the championship pool, the team was tied for third for Quebec's Laurie St-Georges, who they then beat 8–6 in the tiebreaker to qualify for the playoffs.
[10] After defeating Nova Scotia's Kaitlyn Jones 9–7 in the semifinal, they lost a tight 9–6 final to Alberta's Selena Sturmay, settling for silver.
[11] Also during the 2018–19 season, MacMillan skipped the Douglas Royals to a gold medal at the CCAA/Curling Canada College Curling Championships, defeating MacEwan University 7–5 in the final.
[16] At the 2022 British Columbia Scotties Tournament of Hearts, the team qualified for the playoffs through the A Event, defeating defending champions Corryn Brown in the process.
[20] Later in the fall, Team Grandy competed in the 2022 Tour Challenge Tier 2 event where they went 6–1 to claim the championship title, defeating Jessie Hunkin 8–2 in the final.
[22] Entering the 2023 British Columbia Scotties Tournament of Hearts as the number one seed, the team lost both the A qualifier and the 1 vs. 2 game to the Corryn Brown rink.
[28] They continued this momentum into the 2024 British Columbia Scotties Tournament of Hearts where they dominated the field, finishing the round robin a perfect 7–0.
They then defeated Team Brown in both the 1 vs. 2 game and the provincial final to secure their berth at the 2024 Scotties Tournament of Hearts.