The Victoria native had reached the podium in the 800 at each of the last three world championships, but his heat time of 7:50.28 Tuesday left him in 10th place overall, 0.58 seconds out of the top-eight position needed to advance.
Cochrane finished second in his heat behind defending champion Sun Yang of China and sat in third position with one heat left. But a surprising seven swimmers went under 7:50 in the final heat, eliminating the Canadian.
"The distance events are getting faster and faster. The heats felt more like a semifinal," Cochrane said. "I thought I did enough to get into the final and I'm obviously disappointed."
Cochrane was coming off a bronze in the 400 on Sunday that was the first-ever medal for Canada in that event at the world championships. It was a pleasant surprise in that the 400 is considered Cochrane's third-best event.
He'll now turn his attention to his preferred distance, the 1,500, with heats going on Saturday.
Cochrane has won the world silver in the 1,500 three times running, and also owns a pair of Olympic medals at that distance.
Canadians fall short
Day 3 of the meet ended early for Canada as none of its swimmers were able to advance through their heats.
Edmonton's Richard Funk finished 20th in the men's 50m breaststroke with a time of 27.71. Cameron Van Der Burgh of South Africa swam 26.62 to break the first men's world record at this meet after four women's marks were set over the opening two days.
In women's 200m freestyle, Emily Overholt of West Vancouver finished 21st and Montreal's Katerine Savard was 26th.
Wednesday's heats will include Pan Am Games champion Audrey Lacroix in the women's 200m butterfly, and Santo Condorelli in the men's 100 freestyle, where his time of 47.98 is the fastest in the world this year.