The defending champion in the event after his triumph in Shanghai two years ago, Magnussen was a hot favourite for gold in London last year but finished one hundredth of a second behind American Nathan Adrian and had to be content with silver.
Adrian, sixth in Shanghai, was quickest in Wednesday's qualifying ahead of compatriot James Feigen, with Magnussen fourth fastest behind Marcelo Chierighini of Brazil.
"That was all right," Magnussen said.
"I saw the first semi and it wasn't very fast so I thought I'd just try and go out a bit easier in the first 50 and I felt like it was reasonably controlled and it felt pretty good," added the 22-year-old.
"I just really want to control my own race tomorrow night and I'm looking forward to racing on the big stage again."
There are four more finals on Thursday.
American Ryan Lochte can equal Michael Phelps in winning three consecutive world titles in the 200 metres individual medley, while Spain's Mireia Belmonte Garcia could claim a first gold for the host nation after going quickest in Wednesday's 200 metres butterfly semi-finals.
Fu Yuanhui of China was fastest in qualifying for the 50 metres backstroke, with compatriot and world record holder Zhao Jing lurking in fourth, and favourites Australia and United States face off in the 4x200 metres freestyle relay.