Ledecky, who won the 400m free on the opening day of the trials on Sunday, clocked 1min 54.88sec to finish ahead of Missy Franklin (1:56.18) -- a quadruple gold medalist in London who booked her first individual Rio berth after failing to make it in the 100m backstroke.
The 200m freestyle is the latest event to be conquered by Ledecky, who won it at last year's world championships along with the 400m, 800m and 1,500m free -- the last of which is not an Olympic event for women.
She has now secured her spot in two of the three freestyle events in which she's expected to contend for gold in Rio, with only the 800m berth left to lock up.
Leah Smith, Rio-bound as runner-up to Ledecky in the 400m free, finished third, while London Games gold medalist Allison Schmitt finished fourth, 1.84sec behind Ledecky.
That's good enough to send her to Rio as part of the relay pool. But she became the fourth 2012 gold medalist to fail to qualify to defend an individual title in the first four days of the trials.