McIlroy littered six birdies with a couple of dropped shots on the back nine as he began his build-up for next week's US PGA Championship.
"I was a little rusty out there, just hadn't played in a week and I hadn't really done a ton of practice," said McIlroy, whose only outing since his US Masters triumph was a tie for 12th in the defence of his Zurich Classic title with close friend Shane Lowry.Â
"I drove the ball pretty well. I certainly can tidy a few things up, but overall a decent day."
The US PGA Championship takes place at Quail Hollow, where the world No.2 has won four times, which has forced the Truist Championship - formerly the Wells Fargo Championship - to move to Philadelphia Cricket Club.
Mitchell took full advantage of the different surroundings on Thursday, setting a new course record with a nine-under-par 61 as only eight of the 72-strong field failed to break par.
After opening with five straight pars, the 33-year-old birdied nine of the remaining 13 holes to lead by one from Denny McCarthy of the US.
Three more Americans in Collin Morikawa, Rickie Fowler and Akshay Bhatia and Austria's Sepp Straka were a shot further back with Lowry and US Ryder Cup captain Keegan Bradley among a group on six-under-par.
Sweden's Alex Noren marked his return to action for the first time since August following neck and leg injuries, opening with a 67.
Adam Scott (68) is the best-placed Australian at two under, while Min Woo Lee and Cam Davis are one stroke further back.
With AAP.
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