Steve McClaren won the League Cup and reached a UEFA Cup final during his tenure as Middlesbrough boss and, in his first managerial game against his old club, he saw his new team emerge somewhat lucky winners as the Teessiders' brave resistance was finally broken.
Chris Martin had given Derby a 33rd-minute lead with his 10th goal of the season after Boro's Curtis Main was dismissed.
Dean Whitehead restored parity, before Sammon's late strike clinched a 2-1 win and the spoils for the Rams.
In the night's other game, Ross McCormack fired his 14th and 15th goals of the season as Leeds moved to the fringes of the play-offs places with a 2-0 victory over managerless Wigan, who slipped to a fourth straight defeat.
Leeds striker McCormack bundled home his seventh goal in five matches in the 15th minute and then saw his 77th-minute free-kick curl inside the far post for a game-clinching second to lift Brian McDermott's side up to seventh place in the table.