He also ran his fastest-ever split time in the 4x400m relay - 45.42 seconds. The old British record time in the mixed event, which was set in 2019, stood at three minutes 12.27 seconds.
Great Britain posted the fourth fastest time overall across two semi-final heats – and missed out on third spot by just 0.21 seconds after Thompson was pipped on the final stretch.
The race was won by Poland, with the Netherlands in second and Jamaica third.
Tokyo 2020 is the first time a mixed relay event will be held at an Olympics, with the Great Britain team made up of athletes selected from the women’s and men’s 4x400m relay squads.
Cameron Chalmers, Zoey Clark and Emily Diamond made up Great Britain’s 4x400m record-breaking mixed relay team on Friday.
The line-up could be changed for the final, however, which will take place at the Japan National Stadium in Tokyo at 1.35pm UK time on Saturday July 31.
Away from athletics, Worksop-based Thompson works full time as an accountant for Mike Ashley’s Frasers Group in Derbyshire.
He anchored GB Men’s 4x400m Relay team to bronze at the European Indoor Championships in March – and is hopeful of securing a spot in the men’s 4x400m relay team when that event sarts next Friday.