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The 27-year-old Birchfield Harrier's time of 9.96 seconds was also the quickest time by a Briton this year.
Of Britons, only Linford Christie and James Dasaolu have run 100m faster.
Fearon, who will not be at Rio 2016, was part of the four-man bobsleigh team that came fifth in the 2014 Winter Olympics.
The Coventry-born athlete was part of the GB 4x100m relay squad at the 2013 World Championships but did not compete at the event.
He had improved his 100m personal best from 10.10 to 10.04 at a meeting in Loughborough two weeks ago.
Fearon becomes only the seventh British man to post an official sub-10-second time for the distance.
Despite his impressive performance Fearon will not be competing at the Olympics in Rio.
GB's male 100m representatives have already been selected, with Dasaolu and Ujah joined by James Ellington.
Dasaolu's personal best is 9.91secs, achieved in 2013, while Ujah ran 9.96s in 2014. Ellington's personal best is 10.04s.
In an interview with Athletics Weekly after Saturday's race, Fearon did not rule out pursuing his track ambitions before next year's Athletics World Championships in London, but said the 2018 Winter Olympics remained his focus.
"It's a year before our Olympics so it's up to [British Bobsleigh]," he said.
"I'm their athlete so if they give me the freedom to go out and run, and if I'm in a position where I could make it, they'll allow me to go and do it. I'll work with the bobsleigh guys and see what we can put together.
"My focus is really on the bobsleigh at the Winter Olympics. We missed out on a medal in Sochi so I'm very focused on going and redeeming that and hopefully getting that medal that we should have got."
Fearon is not alone in taking his sprinting talents to the bobsleigh track.
British sprinter Mark Lewis-Francis, who won Olympic gold as part of the 4x100m relay team at the 2004 Athens Games, joined the GB Bobsleigh set-up last year in a bid to reach the Pyeongchang 2018 Winter Olympics.
Fellow 2004 Olympic gold medallist Jason Gardener also competed in the 2008 GB Bobsleigh championships, while other sprinters who have made the switch include Simeon Williamson, Marcus Adam, Allyn Condon and Beijing Olympian Craig Pickering.
Bobsledder Joel Fearon ran the joint-third fastest 100m by a Briton at the English Athletics Championships on Saturday.