Mills, who won the Canadian Tour Order of Merit in 2003, established a new 54- hole tournament record on Saturday. His 198 topped Pete Morgan's former record from 2002 by three shots.
Johnson shot a 2-under 70 and is through three rounds at 14-under-par 202. Ken Duke posted a 4-under 68 on Saturday and is alone in third place at minus-12.
Mills, who trailed by three to start the third round, birdied four of his first six holes on Saturday and the last birdie at six tied him for the lead at 13-under par.
He drained a 35-foot birdie putt at the 11th to take over the top spot on the leaderboard. Johnson matched him with a birdie at 10, but Mills caught fire again to provide his comfortable cushion.
Mills birdied the par-5 12th and added his third birdie in a row at 13. He birdied the next par-5, the 16th, and closed with a birdie at No. 18 to put himself in great shape for his first Nationwide Tour win.
'I made some putts when I needed to make them,' said Mills, who leads the tournament in putting. 'It just seemed like everything was clicking.
'It's just a new experience for me. It's all part of it I guess. I'm just looking forward to the final round.'
Doug LaBelle II carded a 5-under 67 on Saturday and is tied for fourth place with Erik Compton, who shot a 3-under 69 in round three. The duo is knotted at 10-under-par 206.
Tom Johnson, who was alone in second after two rounds, struggled to a 2-over 74 in the third round. He shares sixth place with Cliff Kresge (67), Nick Cassini (67), Dave Christensen (67), Bryce Molder (71) and Jason Buha (71) at minus-9.