Williamson, 34, becomes a two-weight British champion having also won the belt at light-middleweight – and put a torrid run of results behind him after three losses in his past five fights.
Simpson, meanwhile, fell to his first professional defeat in 19 fights and after being serenaded with “Yorkshire, Yorkshire, Yorkshire” chants by a raucous crowd for most of the night, quickly jumped on the microphone to quell potential trouble in the audience.
The Barnsley native sells most of his tickets personally among his fans and paid tribute to them after the loss.
Simpson was ahead on the judges’ scorecards when he was stopped in the 10th.
“I thought I was winning most of the rounds. It was always going to be a tough fight with Troy and I got caught with a very good shot,” he said.
“I want to say thank you to everyone who turned out tonight and is still here now.
“I have just been put down and everyone is still here, cheering my name. I will not forget that.
“We’ve got a rematch. I will get back to work, come back and correct this. Thank you for sticking with me. I will be back.”
A rematch could be on the cards in 2026 and Williamson said he welcomed “all comers” in the new year.
“This one is for my baby boy at home,” Williamson said.
“He comes up to me every single day and says ‘and the new…’ I don’t even have to tell him.
“I promised him I was bringing the belts back to him in the hotel tonight and I am absolutely over the moon.”
Williamson added: “This fight has changed my life. I have always wanted my children to have a better upbringing than I had.
“I deserve that tonight with a performance of a lifetime. I have changed mine and my kids’ lives for the better.”