Johnson finished with a time of 44.46 seconds, ahead of Britain's Mark Richardson and Jerome Davis of the United States.
"I felt good, everything's right where it should be," Johnson said.
Though well off his world best of 43.18, set at last year's World Championships in Seville, the time was quicker than Johnson's 45-second effort earlier this week in Berlin.
Late last month, in his first race back after an injury at the U.S. Olympic trials in Sacramento, Johnson ran an impressive 44.07.
Johnson, a double gold medal-winner in the 200 and 400 at the 1996 Atlanta games, is slated to run only the longer event in Sydney, after pulling up with a leg injury in the July trials in the 200 and failing to qualify.
"I feel more relaxed without the pressure of having to run both races," Johnson said. "I think I'll be able to enjoy the Games more this time."
Kenya's Noah Ngeny, who last year set the world record at the unusual 1000 meter distance at this meet in central Italy, won the 1,500 meters in a time of 3:30.42. Ngeny's countryman Benjamin Kipkurui finished second in 3:30.73.
In the men's 100, Ghana's Abdul Aziz Zakari won in a time of 10.13 seconds, finishing ahead of Americans Brian Lewis and Dennis Mitchell.
Nigeria's Mercy Nku was a double winner on the women's side, nabbing both the 100 and 200. Nku finished in 11.20 in the 100, edging Petya Pendareva of Bulgaria. Nku nipped Jamaica's Juliet Campbell by one-hundreth of a second in the 200, finishing in 22.96.
Italy's Andrea Longo won the men's 800 in 1:43.74. Paolo Dal Soglio nabbed another gold for the Italians, taking the shot put with a top effort of 68 feet, 1 inch.
In other men's events, Russia's Va Strogalyov won the pole vault at 18-4 1/2, and Francis Obikwelu won the 200 in 20.21.
Among the women, Ana Guevara of Mexico won the 400 in 50.58, and Surinam's Letitia Vriesde cruised in the 800 with a winning time of 2:01.42.
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