The 34-year-old was unable to reach the exhilarating heights of his opening round, but still produced six birdies against a single bogey at Trinity Forest Golf Club in Dallas.
At 15 under par Leishman leads by one from American Aaron Wise - the 21-year-old who finished second on his last appearance at the Wells Fargo Championship.
Leishman, who won twice on the PGA Tour last season, started on the back nine and holed a 55-foot birdie putt at the 12th before picking up another shot on the 14th.
Finding two bunkers cost him a dropped shot at the 15th, but four birdies in five holes from the first kept him ahead of the chasing pack.
"I'll certainly take that - when you shoot a really low one like I did yesterday then it's hard to back it up," Leishman told the PGA Tour website.
"There probably was a few more out there but I holed a couple of good putts."
Wise fired six birdies and an eagle in his round of 63, with compatriot Brian Gay a shot further back on 13 under after a round of 62.
Watch the AT&T Byron Nelson on Sky Sports from 5pm on Friday