Seeley began his motorcycle career as an apprentice for Vincent Motorcycles. He then raced in the British and world championship sidecar competitions between 1961 and 1967. His best result as a competitor was a second place finish in the sidecar competition at the 1964 Isle of Man TT.
When the AMC motorcycle firm ended production in 1963 due to financial problems, Seeley purchased all the tooling and spares for their AJS and Matchless motorcycle brands. After retiring from competition, he concentrated his efforts on designing and constructing Seeley motorcycles powered by AJS and Matchless motorcycle engines. During the late 1960s and early 1970s when almost all the major motorcycle manufacturers had pulled out of Grand Prix racing, the Seeley-designed chassis became the race bike of choice for privateer racers of the era. John Cooper won the 500cc class at the 1968 North West 200, while Brian Ball finished second to Giacomo Agostini in the Senior TT at the 1968 Isle of Man TT, both riders on a Matchless-Seeley. At the 1969 North West 200, John Blanchard and Brian Steenson placed first and second in the 500 class aboard Seeley-designed motorcycles. In the 1969 Isle of Man TT, Seeley placed four of his machines in the top ten in the Senior TT race with third, fourth, sixth and seventh place finishes. His best result as a constructor came in 1970 when Tommy Robb rode one of his machines to a fourth place finish in the 500cc world championship. Seeley later used Japanese engines in his chassis, most notably using a Suzuki T500 engine for Barry Sheene in 1971. Sheene used the bike to win the British national championship and declared it the best-handling motorcycle he had ever ridden.
His designs proved so successful in competition that he earned a reputation as one of the best motorcycle frame designers in the industry. In the 1970s, Seeley moved to automobile racing when Bernie Ecclestone hired him to work in the Brabham Formula One team.
As vintage racing increased in popularity, Seeley's motorcycles experienced a resurgence with a victory at the 1988 Manx Grand Prix. Since then, Seeley machines have won twelve times in vintage class racing at the Manx Grand Prix.
- Seeley, Colin (2006). Colin Seeley; Racer and the Rest Volume One. Books.Google.com. Retrieved 8 June 2011.
- "Colin Seeley Isle of Man TT race results". iomtt.com. Retrieved 11 June 2011.
- "North West 200 race results". northwest200.org. Retrieved 11 June 2011.
- "1968 Isle of Man Senior TT race results". iomtt.com. Retrieved 10 August 2012.
- "1969 Isle of Man Senior TT race results". iomtt.com. Retrieved 11 June 2011.
- "1970 500cc world championship results". motogp.com. Retrieved 11 June 2011.
- "Memorable Motorcycles Seeley Suzuki". motorcycle-usa.com. Retrieved 11 June 2011.
- "1988 Manx Grand Prix Senior Classic race results". iomtt.com. Retrieved 11 June 2011.
- "Seeley race results". iomtt.com. Retrieved 11 June 2011.