Mahon pulled the strings against his former club in his fourth outing for Boro, having joined on loan from Notts County last month, while Marcus Haber's 73rd-minute goal sealed the victory.
"Gavin Mahon showed all of the experience and qualities that I think we've lacked in previous weeks," said Smith. "I thought from start to finish we were the better team.
"I thought Darius (Charles) was absolutely immense at centre back. And, not just because he scored the goal, Marcus Haber's performance up front gave us a real platform to work with and, of course, on top of that you've got many guys who chipped in with good showings.
"It reinforces what we know about the team. This game never ceases to amaze you.
"We played against four sides who, in the league table, were below us but, in all honesty, didn't get an awful lot out of and here, I thought we were far better than a side who are probably one of the favourites to be promoted.
"The guys were fully on board and I knew they were going to compete for every scrap of grass on the field and they did that, and more."
A first defeat in six saw Brentford miss out on the chance to move into the top three and boss Uwe Rosler admitted the buck stopped with him, suggesting his side were outmuscled by Stevenage.
"First of all the formation we set up didn't work out as good as I hoped," he said. "We had three guys in central midfield and I thought we would get control of that.
"I take a fair share of blame for the formation. It didn't work out as good as I hoped."
- Sports & Recreation
- Marcus Haber
- Gavin Mahon