The Missile edged Americans Jimmy Feigen and Nathan Adrian, who settled for silver and bronze. The winning time was 47.71 seconds.
Russia’s Vladimir Morozov went out strong, leading Adrian at the turn with Magnussen in fifth. But the Australian was stronger on the return lap, touching ahead of Feigen (47.82) and Adrian (47.84). Morozov faded to fifth.
Magnussen hopped on a rope and flexed for the crowd, taking a step toward making up a disappointing performance in the London Olympics. He was favored in the 100 free but lost to Adrian by hundredth of a second, and didn’t even make the final of the 50 free.
Earlier, Ryan Lochte returned to the top at the championships, cruising to victory in the 200-meter individual medley.
Lochte trailed Brazil’s Thiago Pereira after the butterfly and backstroke legs, but the American took control on the breaststroke and won going away with a powerful freestyle finish. The winning time was 1 minute, 54.98 seconds.
Japan’s Kosuke Hagino took the silver in 1:56.29, while the bronze went to Pereira in 1:56.30.
Lochte got off to a disappointing start in the championships, finishing fourth in the 200 freestyle. But, in the third of an expected seven events at Barcelona, he has another world title.
There wasn’t much of a celebration. Lochte still has to swim the semifinals of the 200 backstroke later Thursday.