He was brought up in Kirtling near Newmarket, on the Cambridgeshire/Suffolk border, where he still lives, and started riding out for James Fanshawe aged 12.[1] He served his apprenticeship with Michael Wigham in Newmarket and won with his first professional ride on Broughton Knows at Lingfield on 1 October 2004.[2]
In his early career, he became first jockey to Walter Swinburn until Swinburn quit training.[1] For Swinburn, he won the prestigious handicap, the John Smith's Cup, at York in 2006.[3] Sprint handicapper Out After Dark also gave him some early big victories, including the 2005 Portland Handicap at Doncaster.
Over the next few years, he steadily picked up more winners. He rode over a hundred winners in both 2011 and 2012, and became the 2012/13 all-weather champion,[1] with 91 winners, after finishing third the previous two seasons.[2] In 2011, he also built a partnership with top-class miler Excelebration,[2] riding him four times and winning two Group 2s.[4]
It would be a few years before he won another Group 1, but in 2014, he won three valuable handicaps - the Lincoln on Ocean Tempest, the Epsom Dash on Caspian Prince, and the Royal Hunt Cup on Field of Dream[7] - and finished fifth in the jockeys' championship.[8]
In 2016, still battling to establish himself among the top jockeys in Britain, he narrowly won another Group 1 sprint - the King's Stand Stakes at Royal Ascot on Profitable, trained by Clive Cox. He missed the birth of his first child in order to take the ride; girlfriend Megan gave birth 45 minutes before the first race.[9] The next day, he won the Prince of Wales's Stakes on My Dream Boat.
[10]
In June 2021, Kirby won his first British Classic, the Epsom Derby, on the Charlie Appleby-trained Adayar.[13]
The ride came as a late jockey change after Kirby lost his original ride, on the better-fancied John Leeper, to Frankie Dettori, who had become available following the withdrawal of a number of Aidan O'Brien's horses.
He is known for being one of the tallest jockeys in the weighing room.[9]