In 2014 he joined Carlin Motorsport to compete in the FIA European Formula 3 Championship, where he collected multiple podiums.

Dennis debuted in the 2020–21 Formula E World Championship driving for the BMW i Andretti team alongside Maximilian Günther.

After a red flag in the final race of the season in Berlin which saw 2 title contenders eliminated on the start line.

[12] At the season opener in Mexico City, Dennis managed to qualify second despite damaging his front wing during the quarter-final session, which he was unable to change due to Parc fermé regulations.

[13] He used his position to overtake poleman Lucas di Grassi in the opening stage of the race, after which the Briton created a gap, taking victory with a margin of eight second and becoming the first winner of the Gen3 era.

[18][19] At the Hyderabad ePrix, Dennis would miss out on the points for the first time that year, being the victim of an error from René Rast, who collided with the back of the Andretti driver in the closing stages of the race.

[20][deprecated source] Misery would pile on in Cape Town, where, having been given a drive-through penalty for under-pressured tyres, the Briton ended up 13th.

[21] In São Paulo, Dennis crashed into the back of championship rival Pascal Wehrlein after contact with Dan Ticktum, causing him to once again not score points and take only a fifth DNF in Formula E. The Brit once again failed to score at the following race in Berlin, being involved in a crash with António Félix da Costa due to a brake failure, though he was able to come back with a runner-up finish on Sunday.

[22][23] This began a podium streak which continued through the races in Monaco, Jakarta, and Portland, edging himself back into the title battle.

[29] During the first race in Diriyah, Dennis battled past Jean-Éric Vergne before extending his lead, scoring the fastest lap and taking the win in dominant fashion, ending up 13 seconds ahead of the Frenchman.

He was chosen to participate in the Friday free practice session (FP1) at the 2023 Abu Dhabi Grand Prix for Red Bull.