Albion suffered a 1-0 defeat to Derby County in the Sky Bet Championship on Saturday afternoon.
The match, marking 125 years since the Baggies’ first at The Hawthorns, was decided late through Andi Weimann’s 84th-minute winner.
The hosts created their first chance inside five minutes - Jacob Zetterstrom saving Isaac Price’s early volley.
Despite Derby’s Ben Brereton Diaz flicking a header well wide, it was the Baggies who carried the main threat without opening the scoring.
Aune Heggebø and Callum Styles both saw shots blocked as Mason’s men prodded and probed a stubborn County side, while Price also clipped a cheeky one onto the roof of the net. However, it remained goalless come half-time.
The teams exchanged chances straight after the restart, with Nat Phillips involved at both ends. The big defender couldn’t quite guide his header or a long-range drive home - but did brilliantly block Brereton Diaz’s goal-bound strike.
Captain Jed Wallace went extremely close to breaking the deadlock, hitting one inches past the post, before the head coach turned to his bench for the final stages.
Substitute Samuel Iling-Junior instantly set-up Heggebø, who nodded over, and then Molumby had an effort stopped by Derby’s goalkeeper.
Six minutes from time, though, County stole it when ex-Baggie Weimann converted a low effort.
Jayson Molumby rattled the crossbar as Albion attempted a late equaliser, yet it never arrived and the visitors earned all three points.
Albion: Griffiths, Campbell, Phillips, Mepham, Styles, Wallace (Iling-Junior 73), Molumby, Mowatt (Collyer 73), Johnston (Maja 83), Price, Heggebø.
Substitutes: Wildsmith, Wallis, Gilchrist, Taylor, Diakite, Bostock.
Attendance: 25,596 (2,526).