The 17-year-old finished strongly to win in 21.33 seconds from Ben Snaith, in second, and third-placed David Lima.
"I didn't even think I would make the final so to get the gold medal is literally breathtaking," said Harries.
Adelle Tracey, 22, caused an upset by winning the women's 800m, as Leah Barrow was second ahead of Lynsey Sharp and Jenny Meadows.
Tracey, who was one of six teenagers to light the Olympic flame at the 2012 opening ceremony, said: "I'll be so happy to wear my first senior vest and to do it so early in the season is amazing.
"I'm working part time as a freelance hair and make-up artist and it's good because it fits around my training. I do prosthetics and model making as well, so it's very diverse.
"I did Lenny Waite's wedding this summer and Andy Vernon's wife and Gemma Hillier-Moses (all fellow athletes), because it works very well with training, it's just one morning. But I've done stuff for Dr Who and Top Gear."
The event is being used to select the Great Britain team for the World Indoors in Portland from 17-20 March.
