Lamah appeared to level proceedings with three minutes remaining, only for Mason to rule the goal out due to a perceived offside, despite the ball deflecting off of two West Brom players beforehand.
The decision meant that, after a goal from Romelu Lukaku and an own-goal from Jonathan de Guzman had cancelled out Luke Moore's opener, Swansea came away with nothing from a fiercely-contested tie.
"How on earth can you disallow a goal when the ball comes off an opponent? It's like giving an offside after a throw-in! Everybody knows you cannot be offside after a throw-in," he told reporters.
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"From the defender, to the goalkeeper and back to Roland. So how on earth can you disallow that goal? The 'goal' should have been the equaliser, which I think would have been a fair result looking at the game overall."
West Brom manager Steve Clarke, meanwhile, admitted that the goal should have stood, though he felt his team earned their "lucky break", having deserved a win.
"We got a lucky break. It should have been given. It was a goal," he added.
"I think it came off Ben Foster last so that is fortuitous. But sometimes when you put in the effort, energy and quality we did into that game... I think we earned that little break.
"We deserved to win the game. Don't be writing down that we didn't deserve to win the game. I'm delighted because we came from behind. We don't do it very often and in the Premier League there are not too many teams that can do it."