Burnell and teammate Daniel Fogg were in the leading pack throughout the four circuits of the 2.5km course at Port Vell marina with both British swimmers taking the lead at various stages during the race.
The jostling in a bunched field took its toll on Fogg in the final 2km as he drifted back but 20-year old Burnell kicked on to clock 1:49.30.6 and finish less than 20 seconds behind Greece’s gold medallist Spyridon Gianniotis (1:49:11.8).
"That was unbelievable," said Burnell. "My first major champs and fourth open water overall. I didn't know what to expect because I've never been in a field the quality or size of this one.
"I just threw everything at it, I threw the kitchen sink at it. I was leading and round third for probably two to three laps. I think I probably didn’t have the experience or knowledge to put myself in the correct position for the last lap which I'll learn from and take that forward.
"I had no expectations, I just thought I'll see what I can do and just enjoy. And I did, I really enjoyed it."
"It's his first World Championships and a great result would have been top 25," added Kevin Renshaw, coach to both athletes at British Gas ITC Loughborough.
"Top 20 would be outstanding and top 15 would be a miracle so for Jack, I'm absolutely delighted.
"He swam a great race, a confident race. He's never been in a field of that quality or of that size. Everything we talked about in the last few days leading in to that race, Jack did."
Fogg, who was fifth in the 10km race at London 2012 last year, ultimately finished 38th in 1:50.29.0.
“I’ve felt really good the last few days in training,” said Fogg, who will compete in the 800m and 1500m Freestyle pool events later in the meet.
“I had a great start and was exactly where I wanted to be going up the first front straight.
The field was so strong and bunched up a lot but I got myself back up into a good position with a lap to go.
“I started to kick and the pace really picked up and I couldn’t really go with it with 2km out so I couldn’t be where I wanted to be down the back straight.
“I had nothing in the arms, nothing in the legs, nothing at all to give me that sprint at the end.”