The Texan, who set a world record of 19.66sec at the United States trials last month, finished well clear of Trinidad's Ato Boldon, who recorded 19.94. It was a convincing riposte to those who suggested his confidence may have been affected after losing his unbeaten 38-race sequence in Oslo to Fredericks.
Johnson's performance uplifted a meeting which earlier had to suffer the withdrawal of Moses Kiptanui, who had a cold; the world 100 metres champion Gwen Torrence, who had an allergic rash; and Britain's Kelly Holmes, who was suffering from sinusitis.
While Johnson's sequence of success may have been ended, that of Jonathan Edwards - which stretches back to June 1995 - continues. The world triple jump champion looked relaxed and confident once again as he won with 17.29 metres, and seemed to jump well within himself. His British team-mate Francis Agyepong maintained his recent encouraging form with third place in 16.57.
Tessa Sanderson, preparing for her sixth Olympics at the age of 40, finished sixth in the javelin with 60.76 metres. Respectable stuff - but she was clearly dissatisfied and is seeking to add two or three metres to that distance to give herself a better chance of reaching the final in Atlanta.
Tony Jarrett's Olympic preparations were disrupted when he was disqualified from the 110 metres hurdles after registering two false starts. The world silver medallist walked away from a top-class field with a dazed look on his face after the starter had ruled he had been responsible for the third false start in the event. Jarrett had held up the race on two other occasions, first to remove something from his eye and then to do up his shoelace. It was a nervy performance from him.
Colin Jackson remained impassive throughout the delays, and when the race got away at the sixth attempt he started well enough to remain in contention with the world champion, Allen Johnson, on his right over the first four hurdles. Both faltered at the sixth hurdle, crashing down their barriers, but the American recovered better and pulled away to win in 13.25sec. Britain's world record holder, still searching for his best form after suffering tendinitis in his right knee recently, finished raggedly but claimed second place ahead of Cuba's Amilio Valle.