The Englishman, whose only European Tour victory came 12 years ago, shot a 3-under 68 at the Jack Nicklaus-designed Golf Club at Harbor Shores.
John Cook was alone in second at the first Champions Tour major event of the year, having shot a 69. The only other players under par were Jay Haas, Jeff Sluman, J.L. Lewis, Steve Pate, David Frost and Jim Carter, all at 1-under 70.
Chapman is a member of the European Senior Tour, which has had only one tournament thus far this year.
"This is only my fourth real competitive round this year," Chapman said. "It's a long way to come, but I'm very pleased with the way things are going."
Defending champion Tom Watson had to pull out of the event due to a wrist injury.