The 22-year-old clocked 19.95 seconds representing Louisiana State University at the Southeastern Conference (SEC) Championships in Tuscaloosa, Alabama.
John Regis set the British record of 19.94 in 1993, while Adam Gemili is the only other Brit to go sub 20.
Mitchell-Blake told BBC Sport his achievement was "overwhelming".
Regis also clocked a time of 19.87 but this was never ratified.
He had already competed in two Olympic Games by the time he ran under 20 seconds.
Mitchell-Blake clocked his time having already won the 100m as well as anchoring his university to victory in the 4x100m at the SEC Championships.
"The 200 is my favourite event," said the 2013 European junior champion, who was visibly emotional when approached for an interview on the track..
He said he wanted to compete for Britain at the Rio Olympics in August but would be focusing on his college career for the next few weeks.
"Then I'll come back to the UK and try to compete for a spot on the team," he added.
Chijindu Ujah and James Ellington have also already run under the 20.50-second Olympic A standard in 2016.
The British Championships take place in June.
Nethaneel Mitchell-Blake missed the national record by one-hundredth of a second as he became only the third Briton to run sub 20 seconds for 200m.