The former Devonport High School for Boys pupil stormed to victory in a time of 13.58 seconds.
He was well clear of his big rival Cameron Fillery, who took second in 13.83, with Harrow’s Miguel Perera third in 14.68.
The win meant King, who has been training in America this year, completed a British double-double, having won both the indoor and outdoor national titles in the last two years.
It was his third senior outdoor British crown and will be a boost to him as he looks to try and achieve his dream of competing at the Tokyo Olympics.
King, who had comfortably won his heat, said afterwards: “I am really happy with that and a very fast time for a cold evening in Manchester. That really sets you up for the big ones.
“I am running well and have improved technically so I am very happy. I am so pleased this was able to go ahead – it has been even different with no hurdle practice in the warm-up but that’s why the first run was so important to actually be able to hurdle. It has been a great event so far.”
The opening day of the 2020 British Championship, which are taking place behind closed doors, also proved successful for Cornwall youngster Sam Brereton, who won bronze in the high jump at the age of just 17. Under-20 champion Brereton leaped 2.15m, which was an outdoor PB and only one centimetre off his lifetime best achieved indoors earlier this year. He only missed out on silver on countback.
City of Plymouth’s Tyler Johnson competed in the 400m heats, but was unable to progress with a time of 48.77 seconds.
The championships continue on Saturday when Erme Valley Harriers triple jumper Sam Trigg and Tamar Trotters’ Mike Wilsmore will be in action. Wilsmore clocked 3:46.57 in the heats to qualify for the final. He was initially disqualified for obstruction but was reinstated.