McCoy stopped Williamson 8 and 6 in the 36-hole final at the Country Club of Birmingham.
The 50-year-old McCoy, from West Des Moines, Iowa, is the second-oldest winner in the event limited to players 25 and older. Randal Lewis won the 2011 tournament at 54.
“It’s a great day for me,” McCoy said. “And I’m very thrilled to finally have won a (U.S. Golf Association) championship. I’ve been trying for a long time.”
U.S. WOMEN’S MID-AMATEUR: In Asheville, N.C., Julia Potter, a recently reinstated amateur from Granger, Ind., won the U.S. Women’s Mid-Amateur, beating Margaret Shirley, of Roswell, Ga., in 19 holes.
EUROPEAN TOUR: In Vilamoura, Portugal, Alviro Quiros headed a group of six golfers tied for the first-round lead in the Portugal Masters with 6-under-par 65s.
Former Augusta State golfer Scott Jamieson is one of nine golfers one shot off the lead after 66s.
LPGA TOUR: In Kuala Lumpur, Malaysia, Ilhee Lee shot 7-under-par 64 to take a one-stroke lead over Brittany Lang after the opening round of the LPGA Malaysia.
TURNING PRO: Amateur teen phenom Lydia Ko plans to compete as a professional in the LPGA’s CME Group Titleholders in October. Ko’s mother, Tina Hyon, told GolfChannel.com that Lydia, 16, will make the trip to Naples, Fla., for the tour’s season-ending event.