Porsche series In 2015, Cullen performed double-duties, partaking in the
Porsche Carrera Cup Great Britain with G-Cat Racing whilst also racing for
VERVA Lechner Racing Team in the
Porsche Supercup from round 4 onwards. He scored a best race finish of eighth in the former and 12th in the latter. After finishing third in the Porsche GT3 Cup Challenge Middle East during the off-season, Cullen returned to the
Porsche Supercup with Lechner Racing whilst also competing in the
Porsche Carrera Cup Germany at
Konrad Motorsport. In the latter, Cullen finished tenth in the standings, being beaten by all full-time drivers, meanwhile he classified 11th in the Supercup with a best result of seventh at the
Red Bull Ring. For his third season in the Porsche world, Cullen remained with Lechner in the
Supercup, but switched to Huber Racing in the
PCCG. He would end up 11th again in the former, whilst improving to ninth place in the latter's standings.
LMP2 venture Cullen switched to prototype racing in 2018/19, driving in the
LMP2 class of the
European Le Mans Series for APR -
Rebellion Racing alongside
Gustavo Menezes and
Harrison Newey. The team ended up ninth in the standings, having finished all six races. At the start of 2019, Cullen took his maiden endurance racing victory in the
24 Hours of Daytona, having driven for
DragonSpeed together with
Pastor Maldonado,
Roberto González, and
Sebastián Saavedra. For the main 2019 season, he remained in the
ELMS, this time partnering
Alex Brundle and
Will Owen at
United Autosports. Cullen and his teammates scored their first podium of the year at
Monza, finishing third, but were unable to match the results of the sister car, finishing eighth in the teams' championship. Cullen made his
24 Hours of Le Mans debut the same year, finishing 14th in class. Without a full-time programme in the
COVID-stricken 2020 campaign, Cullen had to make do with one-off appearances. He raced in two races of the
ELMS with DragonSpeed, drove for
Jackie Chan DC Racing at the
6 Hours of Spa-Francorchamps, and competed at
Le Mans for G-Drive Racing. Cullen underwent another patchy season during 2021, as he raced in two events of the
FIA World Endurance Championship with
Risi Competizione, which included another appearance in
Le Mans - where he and his teammates retired due to engine troubles. The following year saw Cullen step into the
WEC for the entire season, as he drove with new team
Vector Sport in the LMP2 category. Despite scoring a podium at
Monza, Cullen and the team struggled, ending up tenth and second-lowest of the Pro class entries in the standings. Returning for the team's second season in the category, Cullen partnered
Gabriel Aubry as well as fellow silver-ranked driver
Matthias Kaiser for the
2023 WEC season. With numerous technical issues and a collision at Monza, Vector Sport finished tenth overall, once again ranking second-lowest. However, a strong drive by Aubry carried the team towards a seventh place in class at
Le Mans. With the dropping of LMP2 from the WEC after 2023, Cullen returned to drive for Vector in the
2024 European Le Mans Series. Partnering
Felipe Drugovich and
Stéphane Richelmi, the Irishman finished 14th in the drivers' standings.'''''' Of note was a collision he caused at
Barcelona, where an overly ambitious overtaking attempt led to him spinning
Alex García into the gravel, and a self-induced spin at
Imola — though the team recovered well and finished the race in third. He finished fourth in class at
Le Mans, having been partnered by Richelmi and
Patrick Pilet. Cullen remained at Vector for the
2025 ELMS season, where he drove alongside
Pietro Fittipaldi and
Vladislav Lomko. The trio began well in
Barcelona, finishing fourth overall and taking third in the Pro category. Fourth in Pro at the next round in
Le Castellet preceded 12th place in
Imola, a race where the Vector car was turned around late by
Tom Dillmann. Points in
Spa came next, before Cullen and his teammates completed a tidy race at a rainy
Silverstone to claim third overall. ==Personal life==