He was recruited by Fremantle with their second selection, 38th overall, in the
2016 AFL draft. He made his AFL debut in Round 14 of the
2017 AFL season, against
Geelong at
Simonds Stadium, after playing well for Fremantle's reserves team,
Peel Thunder, in the
West Australian Football League (WAFL). Darcy amassed 40
hit-outs in his debut game. Due to his physical size and playing style, he is often compared to
Shane Mumford. A breakout performance during the
2021 AFL season saw Darcy win his first Fremantle
best and fairest award, the
Doig Medal. Darcy played every game in 2021, establishing himself as a premier ruckman.
Western Derby 55 saw Darcy amass a derby record 56 hit-outs during Fremantle's 24-point win over the
West Coast Eagles. He was awarded the
Glendinning–Allan Medal for best on ground. Darcy started the
2023 AFL season in brilliant form, and as of 21 April led the league for both total hitouts (204) and hitouts to advantage (56). Darcy’s strong form was cut short with a hamstring injury in Round 11 against . He made his return in Round 15 during Fremantle's 32-point win over . Already contracted until 2024, Darcy signed a six-year contract extension in December 2023, tying him to Fremantle until at least 2030. Darcy suffered a knee injury in the off-season during Fremantle's practice match against , and as a result missed the first five games of the
2024 AFL season. Sean made his return during Western Derby 58 collecting 13 disposals and 30 hitouts. He played his hundredth game the next week against the . He kicked two goals both courtesy of
50-metre penalties and collected 12 hit-outs against in round eight at the
MCG, before being substituted out of the game in the third quarter with a tight calf. As a result he missed Fremantle's next two games before returning in round eleven against . He was ranked the most efficient ruckmen for hitouts to advantage in the competition as of July, with a return of 15.35%. ==Statistics==