[7] Cassidy began competing in Formula E for the Envision Virgin Racing team starting with the 2020–21 season, partnering Robin Frijns.

[9] However, Cassidy spun out of the lead on the opening lap as a result of a software glitch, before being pushed into the wall and out of the race by Oliver Rowland.

[21] Cassidy finished on the podium at London as a result of a penalty for De Vries and ended the season eleventh in the standings, four positions behind teammate Frijns.

[23] After two events that earned him points finishes, Cassidy scored back-to-back podiums at Hyderabad and Cape Town before missing out narrowly on victory to Mitch Evans at the São Paulo ePrix.

[24][25][26] The Kiwi continued his championship assault in Germany, winning the second race of the Berlin ePrix, narrowing his gap to leader Pascal Wehrlein to four points.

[27] At the next round in Monaco, Cassidy charged from his starting spot of tenth to second within the opening seven laps, before executing a pass on title rival Evans to take victory in the principality.

[28][29] Cassidy struggled in Jakarta, finishing seventh on Saturday before losing out on points on Sunday after damaging his front wing in a self-inflicted accident with Wehrlein, though he would return to form with a win at Portland, having managed his car's energy well in a pandemonious, peloton-style race.

[33][34] Still with good chances to win the title, Cassidy qualified second for the first London race but was promoted to pole after Evans's penalty for the Rome accident had been applied.

[35] During the race, drama ensued: Cassidy, who had been leading the race until his first attack mode activation dropped him back to fourth, became the victim in a teammate collision as miscommunication caused him to rip of his car's front wing, with Buemi, who had been ready to help Cassidy in his title ambitions by letting the Kiwi through, received no communication from his team.

[38][39] Cassidy switched to Jaguar TCS Racing on a multi-year contract, replacing Sam Bird and partnering fellow Kiwi driver Mitch Evans for the 2023–24 Formula E season.