Harrison grew up in Bradford and came to motorcycle racing at the age of 18, entering the sport relatively late compared to many of his peers. His father, Conrad Harrison, was himself a TT competitor who won a Sidecar TT race in 2014, placing Dean within a family tradition of road racing. From 2024, Harrison has been based in Laxey on the Isle of Man, bringing him closer to the course that has defined his career.
Harrison's Isle of Man TT breakthrough came in 2014 when he won the Lightweight TT. After building a reputation for consistency through the mid-2010s, he claimed the Duke Road Racing Rankings championship in both 2016 and 2017, earning the Geoff Duke Trophy for each title. Those championships recognise the most accomplished road racer across the international road racing calendar in a given season.
At the TT itself, Harrison added Supersport and Senior victories in 2018 and 2019 respectively, then in 2025 swept both Superstock TT races to lift his win total to seven. His 2018 campaign was particularly notable: he set a Superbike TT lap record of 16:50.384 aboard a Kawasaki ZX-10R, a time that stands as one of the fastest ever recorded on the Mountain Course. His best outright lap speed reached 134.9 mph, which at the time ranked as the second-fastest lap in TT history.
Beyond the Isle of Man, Harrison built an exceptional record at Oliver's Mount in Scarborough, accumulating 68 solo wins at that circuit. At the Southern 100, held on the Billown circuit in the Isle of Man, he became the third most successful rider in the event's 65-year history with 24 solo wins and three solo championship titles between 2017 and 2019. His North West 200 record includes four podiums, and at the Ulster Grand Prix he has two outright wins and holds a Superstock lap record of 133.835 mph.
Harrison also competes in the British Superbike Championship on short circuits alongside his roads programme, demonstrating a range that few road racing specialists maintain.
For much of his professional career Harrison raced with DAO Racing, a partnership that spanned eight years. At the conclusion of the 2023 season he departed the team, and in November 2023 it was announced that he would join Honda Racing UK for both the British championship and road racing from 2024 onwards.
Harrison represents the steady, methodical school of TT racing — a rider who accumulates results through precision and mechanical sympathy rather than outright aggression. His seven TT wins place him in elite company, and his lap records and championship titles confirm his standing as one of the definitive road racers of the 2010s and 2020s. His combination of short-circuit and road racing activity makes him one of the most versatile British motorcycle competitors of his era.