Shearer was taking charge of the team as Thistle are yet to replace Terry Butcher, and he had to watch his side hold off a strong rally from the Saints, who piled on plenty of pressure but failed to create many chances.
The victory came provided Inverness with a little revenge for a heavy 4-0 defeat in Perth last month.
"I thought the fans were terrific," he said. "They knew we were under some pressure at the end and got behind us which was immense.
"I didn't enjoy the game but I was delighted for the back four because they got rubbished about down there (at McDiarmid Park) last time and they were determined that it wasn't going to happen again today.
"They were backed up by the midfield and of course wee Billy (McKay) who got the goal for us. He didn't do much for the rest of the game but I'll take that kind of striker any day."
Saints boss Tommy Wright was left to rue his side's lack of cutting edge as they failed to regularly test home keeper Dean Brill.
"There was a lack of quality in the final third," he said. "I don't think it was a lack of effort but you have to compliment them on the way they defended and restricted us to very few opportunities.
"In the last third we did okay but we did not get enough chances or shots away. It's always difficult to come here but to lose a goal here early in the manner that we did.
"And I don't think there was a lot between the two teams. The first goal was always going to be vital. We said that before the game and it was the same when we played them there."