Tigers wing Cobden will join Leicester teammates Ben Youngs and Greig Tonks in the starting line-up to play hosts Japan at the Prince Chichibu Stadium.
Cobden, who will make his England Under-20s debut, had not figured in the Six Nations and his call to take part in last month's training camp at Loughborough came out of the blue.
But England had been monitoring his form on loan at Nottingham and after Seb Stegmann was ruled out of the tournament through injury Cobden was added to the ranks at last month's training camp in Loughborough.
He said: “I was at Nottingham on loan and at the start of the season I was just trying to break into the team there.
“But I managed to get a place on the wing and Glen Delaney, the coach there, has helped bring me on a lot and given me the chance to play.
“I scored 15 tries in 21 games at the end of the season and missed a few other games because of an ankle injury. Playing week-in, week-out has been really good for my confidence.
“I knew about the Under 20 set-up from Greig Tonks, who I share a house with, but when I was asked to go to the training camp at Loughborough it was still a massive shock.
“Mark Mapletoft said that if I had a good week I'd be on the plane to Japan and I think I did well in difficult circumstances at Loughborough.
“I came in on the Wednesday and we played a training game on the Wednesday evening, without really knowing any calls or patterns. I just had to learn them all and fit in as best as I could and I think I've done that well.
“I can't wait to play now and I'm really looking forward to it. It's an honour to play for your country and now it's just a question of doing the best I can against Japan.”
Coach Mark Mapletoft said: “I’m really pleased for Jack. He’s had a good season at Nottingham and we’ve been keeping an eye on him.”
The versatile Tonks will start at inside centre while Youngs, who scored three tries in 17 first-team appearances for Tigers last season, is at scrum-half.
Mapletoft added: “It's a big challenge and this is a must-win game at the start of the tournament but we've selected a team we believe can win the game.”
England go on to play Pool B opponents Scotland on June 9 and Samoa on June 13. The knock-out stages take place on June 17 and June 21.
England Under 20 (vs Japan Prince Chichibu Stadium, 11am BST)
15 Tom Homer (London Irish)
14 George Lowe (Harlequins)
13 Luke Eves (Bristol Rugby)
12 Greig Tonks (Leicester Tigers)
11 Jack Cobden (Leicester Tigers)
10 Rory Clegg (Newcastle Falcons)
9 Ben Youngs (Leicester Tigers)
1 Joe Marler (Harlequins)
2 Jamie George (Saracens)
3 Bob Baker (London Wasps)
4 Dan Williams (Gloucester Rugby)
5 James Gaskell (Sale Sharks)
6 Chris York (Harlequins)
7 Josh Ovens (Bath Rugby)
8 Carl Fearns (Sale Sharks)
16 Ben Moon (Exeter Chiefs)
17 James Clark (London Irish)
18 Graham Kitchener (Worcester Warriors)
19 Courtney Lawes (Northampton Saints)
20 Dave Lewis (Gloucester Rugby)
21 Rob Miller (Newcastle Falcons)
23 Henry Trinder (Gloucester Rugby)