The pair moved up from third place after their penultimate dive to score 69.12 on their final attempt and edge Russia into the silver medal position with Germany taking bronze.
It was an unexpected success for the British pair, who were competing in only their second event together, with Londoner Cheng also marking her senior debut in style.
Toulson, 18, the City of Leeds diver from Cleckheaton, said: “It feels amazing to take the gold, I don’t think we were expecting to get a medal.
“It sets me and obviously the rest of the team up really for the rest of the week.
“There is lots of potential within this partnership and we’re both very young so we’re both just excited to see where it can go.”
Cheng, 15, from London added: “We were very focused on our own dives throughout and that enabled us to just concentrate on ensuring that we took off and then entered at the same time.”