Willie Ryan took the initiative half a mile from home and the colt readily lengthened his stride and went clear of his field.
The filly Fairy Heights, 11-8 favourite to become the first of her sex to win the race since the peerless Noblesse in 1962, gave chase. She was staying on at the end, but King's Theatre had a comfortable length in hand at the line.
Three lengths behind, the winner's stablemate Bude kept on into third place, followed by Khamaseen - another doing his best work at the end - and the rank outsider Chickawicka, who ran a blinder for his small stable after threatening a place two out.
King's Theatre is 16-1 favourite for the 1994 Derby with William Hill, and 20-1 joint favourite with Ladbrokes, who bracket him with Peter Chapple-Hyam's Golden Nashwan, an impressive winner of a seven furlongs maiden at Doncaster on Friday.
The successful trainer Henry Cecil has farmed the Racing Post Trophy in its various guises over the years; King's Theatre was his 10th winner in the mile juvenile test. The colt's prize-money put him back in line for his 10th trainer's title, pounds 54,000 clear of the reigning champion, Richard Hannon.
King's Theatre races in the colours of Michael and Carolyn Poland, but that will change next year. The Polands bred the colt from their mare Regal Beauty, but Sheikh Mohammed bought into him before he was born. Poland said: 'We tossed a coin to see which season he would carry whose colours, and I lost.'
Poland, from Upham, in Hampshire, is master of the Isle of Wight Foxhounds, and put off the opening meet of the hunting season so he could be at Doncaster. He bought Regal Beauty for just 5,200 guineas at the great Jim Joel dispersal auction in Newmarket seven years ago. He said: 'She was so cheap because she was barren and had bred nothing at that stage. I thought if I could not get her in foal I could use her as a hunt horse at that price.'
Poland soon changed his plans. Regal Beauty's 1986 foal turned out to be champion juvenile High Estate; and he was followed by useful winner King's Loch. Those performances as a matron earned her a visit to the best consort in Europe, Sadler's Wells, with King's Theatre the result. The next one in the pipeline is a yearling filly by Royal Academy, due to join Cecil next week.
Neville Callaghan had no regrets about supplementing Fairy Heights to the race. The filly was going as easily as any two out and, though she found no change of gear when Cash Asmussen changed his hands, she stayed on well. And Cecil was as pleased with the eye-catching effort from Bude, a little Dancing Brave half-brother to Tenby, as he was with his winner.