The 51-year-old Daley, winless in his brief PGA Tour career, had a 14-under 266 total at Fox Chapel. He earned $405,000 and full Champions Tour status for a calendar year.
Lehman finished with a 69. He was trying to win his second straight Champions Tour major after taking the Regions Traditions in his previous start.
Daley capped the victory with a 15-foot putt on the final hole for his tournament-high 24th birdie.
Couples, fighting chronic back problems all weekend, bogeyed Nos. 15 and 16 to drop out of contention.
Lehman also faltered late in the 90-plus-degree heat, bogeying three of the final six holes.
Before this weekend, relative unknown Daley was perhaps best known for missing out on a PGA Tour card in 2000 when a short putt popped out of the hole during Q-School.
He missed out on his Tour card by a single stroke that year and was a member of the PGA Tour only in 1996 and 1998. He has one top-10 finish in 59 PGA Tour starts, but can add next season's Players Championship to that total after earning an exemption with the victory.
Daley's best finish in 10 previous Champions Tour events was a tie for fourth in the Senior PGA in May. The $405,000 check figures to go a long way toward earning a full tour exemption next season as a top-30 finisher on the money list.
Daley extended his lead to two strokes with a long birdie putt from near the fringe at 10, his second-to-last birdie of the weekend before his capper on 18.
Daley was at least 2 under during all four rounds, gradually working his way up the leaderboard by the end of each round. Daley was two shots off the lead at the end of the first round, one shot back behind Couples after the second and tied with Calcavecchia after shooting a 68 on Saturday.
Daley took sole possession of the lead with a birdie on the 483-yard par-5 second hole, pulling ahead of Calcavecchia and Lehman. Daley also birdied No. 9.
Couples wasn't the same after a second-round 63, shooting 1 over the final two days. His back stiff, Couples was stretching and moving gingerly throughout the tournament.
Coming off a win last weekend at the Montreal Championship, Calcavecchia played the worst of the players in contention Sunday. He did not have a birdie until the back nine.
Jeff Freeman briefly pulled within one of the lead with an eagle at No. 2, but finished with a 74 and ended up 11th at 6 under.