The Englishman carded seven birdies and a single bogey in a stunning 65 - the joint-lowest round of the week so far - to get to 11 under par and overtake Guerrier at the summit.
Frenchman Guerrier began the day with a two-stroke advantage and had pulled three clear by the turn courtesy of three birdies on the front side.
But as he stood still, mixing one birdie with one bogey on the back nine to sign for a third-round 68, Shinkwin birdied the 11th, 13th, 15th and 18th to hit the front.
The duo had clear daylight between them and their nearest challenger - Scotland's Connor Syme, who was on five under after his 68.
Shinkwin has become used to playing well at Celtic Manor, having finished fourth and joint-eighth in his last two appearances there.
And he was a happy man after impressing again at the 2010 Ryder Cup venue.
After producing one of the shots of the tournament to set up a close-range birdie at the 18th, Shinkwin told the DP World Tour website: "I'm delighted, 65 was a really good score today.
"I'm playing really well and didn't make any mistakes and I've come out of it with the lead."