The 20-year-old came into the final round tied for the lead with world number four Cantlay and secured the victory after his American opponent triple-bogeyed the final hole.
Kim finished with a five-under 66 to go 24-under overall in a bogey-free week at TPC Summerlin in Las Vegas.
"I got very lucky on the 18th, I'm not going to lie," Kim told the Golf Channel broadcast. "Patrick played awesome and it was an honour to battle with him and to come out on top, I feel very fortunate.
"I've got to give big credit to Joe (Skovron), my caddie. He really kept me in it and we had a really good gameplan coming into the week, and it paid off."
Victory was Kim's second PGA Tour title in his last four starts, following his triumph in the Wyndham Championship in August.
"It's really amazing," Kim said. "A few months ago, I didn't have any status in the US and now being a two-time winner on Tour, having that place with Tiger, it's an unbelievable feeling for me. It's an honour and definitely a dream come true.
"I've worked really hard, and my team has worked really hard to this point, and I'm just really grateful... I'm having fun playing on the PGA Tour. I'm a five-year-old at Disneyland."
