Brennan grew up in Darlington, County Durham, and started racing with the local club Stockton Wheelers. He came through British Cycling's junior track programme rather than a continental road apprenticeship, and the results that first got him noticed were on the boards. At the 2023 junior world championships in Cali he won the madison alongside Ben Wiggins, son of Bradley, and the individual pursuit, and took silver in the scratch race.
He raced on the road at the same time, spending 2023 with the British amateur team Fensham Howes-MAS Design before joining Visma's development squad for 2024. Visma promoted him to the WorldTour roster for 2025 rather than keeping him at development level for a second year.
His first WorldTour win came on the opening stage of the Volta a Catalunya in March 2025, aged nineteen, in the rain at Sant Feliu de Guíxols, where he brought back a late attacker and beat Kaden Groves to the line. Four days later he won again at Amposta, out of a peloton that had split apart.
At the Tour de Romandie he took the opening road stage on the uphill drag into Fribourg, jumping from second wheel inside the last 200 metres and finishing several lengths clear. The ten-second bonus put him in the leader's jersey ahead of Sam Watson. It was his third WorldTour win of the season.
The Tour of Norway gave him his first stage race overall. He won stage two, then the final stage into a bunch sprint, and the time bonuses put him twenty-eight seconds clear of Victor Langellotti. He took the points and young rider classifications in the same week.
He won stage five of the Tour de Pologne later in 2025, over 206 kilometres from Katowice to Zakopane, holding off Ben Turner, and stage three of the Tour of Britain into Ampthill.









