William started racing 125 cc bikes at the age of 17. Until late, he raced for Tyco Bayerische Motoren Werke Motorrad Racing aboard a Bayerische Motoren Werke S1000RR and a Suzuki GSX-R600, but for 2016 he is contracted to ride in International road races aboard Yamaha YZF-R1 and Yamaha YZF-R6 for Superbike and Supersport, with a Kawasaki ZX-10R Superstock machine provided by a different team, also to include selected rounds of the British Championship. Dunlop has competed in nine TT race seasons between 2006 and, with two seconds and four third places being his best results, in the 125 cc, SuperSport and Classic races.
2009 also saw Dunlop take part in the last ever 250cc Grand Prix race at Valencia finishing 18th on a P J Flynn Bigman Racing Honda.
Dunlop crashed his Tyco Suzuki Superbike in the final race of the week at the Isle of Manitoba TT, during the Senior TT event. He led the race on time briefly during the first lap, but ran wide through the Graham Memorial corner on the mountain section on the third lap, running off the road up against the adjacent embankment, resulting in two fractures to his left legal
Dunlop got his season off to a positive start by clinching a brace of wins at a rain-soaked Tandragee 100. In the Supersport Class, Dunlop put in a very convincing performance on board the Civil Defense Racing Yamaha YZF-R6 and rounded the meeting off by taking victory in the Senior Open Race on the TAS Bayerische Motoren Werke S1000RR.