Liam Craig curled in a stunning second-half winner after his team were out-played for the majority of a scrappy encounter at a soggy Firhill on Monday night.
Striker Kris Doolan missed two great chances for the hosts and Fenlon said: "It was a bit of smash and grab from us if I'm totally honest.
"I thought Thistle were excellent in the first half and played really well but they probably had more chances in the second half. We were a little bit better in the second half and played better once we scored.
"I'm delighted with the three points as it's a difficult place to come. They're a real good side, with a good system and they pass the ball really well.
"They had a lot of possession in the first half but didn't cause us loads of problems in front of goal but they created three or four in the second half and Ben made some terrific saves."
It proved to be a frustrating night for Partick who are still looking for their first win on home soil this season.
Thistle manager Alan Archibald said: "It was a sore one to take. I thought we played a lot of good stuff and created a number of chances.
"What was missing was a clean sheet, at least you take a point from that, that was the disappointing thing. We haven't kept a clean sheet since the first game of the season and if you don't keep clean sheets you don't win games, it's a simple as that.
"It is frustrating obviously but it would be more frustrating if we weren't creating any chances and we were playing really poor football. But we are playing good football and we are knocking it about well.
"I said that to the players there, it would be easier if we had been terrible tonight. I could have come in and had a go at them, and read the riot act but it wasn't the case."