Johnson-Thompson jumped a massive 6.81 metre personal best in Barcelona to win long jump gold while British team-mate Jazmin Sawyers, who arrived in the Catalan capital as the world's top ranked junior, settled for bronze.
Johnson-Thompson's best was marginally wind assisted but the teenager, who will compete alongside Jess Ennis and Louise Hazel as one of Team GB's selected heptathletes, had no complaints.
"I am really pleased with my performance, especially because I didn't expect it," she said.
"I thought I would be in with a shot at a top five, a medal maybe, if I hit the board right.
"I always knew in my head there was a big jump coming and this time though it all came together when it really mattered."
Sawyers, who had entered the competition leading the world rankings with a top distance of 6.64m, was 'hugely disappointed' to settle for bronze, although she acknowledged the importance of her achievement.
"I know it's a personal best and I know I should be happy with a medal – but I want best," she said.
Johnson-Thompson and Sawyers are Britain's fifth medallist at the World Juniors and second gold medallist alongside 100m champion Adam Gemili, who is also part of the Olympic team.