Fulbright, ridden by Silvestre de Sousa, held off the challenge of Mabait to cross the line more than a length ahead of his nearest rival.
Mister Music placed third with last year's winner Boom and Bust in fourth.
"We were running him over six (furlongs) - clearly we haven't got a clue. He's out of handicaps now, it's going to have to be Group races," Johnston said.
De Sousa added: "There was plenty of speed in the race and he's a horse that has won over six furlongs and seven so I had no doubt I'd have the speed to travel whatever the pace was.
"I was just tracking the leaders and he was enjoying being there so I just let him enjoy the race."
Australia's Ortensia finally won in Britain this summer at the third time of asking with victory in the Gordon's King George Stakes. The horse had previously been beaten at Royal Ascot and in the July Cup at Newmarket.