McCarthy had eight birdies – four in a row from the ninth, and five in six holes – in a flawless round to add to previous scores of 68 66.
He has risen to ninth place in the Race To Amendoeira, two spots behind Hull’s Steve Robins.
Corhampton’s Neil Raymond birdied the second play-off hole at Cumberwell Park to fend off John Henry (Clydebank) to win the HotelPlanner.com EuroPro Tour event.
McCarthy was one of six Yorkshire players to finish inside the top 14.
Rotherham’s David Booth tied for sixth after rounds of 66 67 66, grabbing five third-round birdies with no blemishes on the card.
Hillsborough’s Joe Dean went one better on the final day, with six birdies and no bogeys for a 65 that, along with 66 69 on the first two days, earned him a share of 10th place.
Bondhay’s Dan Wasteney (68 68 66), Huddersfield’s Nick Marsh (65 70 67) and Walton Range/Howley Hall’s Daniel Gavins (68 66 68) were in equal 14th place.
Wasteney had six birdies and a bogey on the final day while Marsh, after 10 straight pars, had two birdies, a bogey at 15, and then an eagle-birdie finish.
Walton Hall’s Daniel Gavins had three birdies and no bogeys in the last round.