The Black Cats produced a blistering first-half display without reward, with Dan Ballard's goal ruled out by VAR, Enzo Le Fée hitting the post, and Noah Sadiki and Thomas Meunier both seeing efforts cleared off the line.
Brentford took the lead after the break through Janelt, before being denied a second themselves when VAR ruled out an effort following a foul by Jaidon Anthony on Robin Roefs. Le Fée converted from the penalty spot in the 82nd minute to earn Sunderland a share of the points.
Brentford boss Keith Andrews felt his side let a winnable game slip.
"We caused ourselves a lot of problems. We were better in the second half. We started fighting more, did the ugly side of the game and played with a bit more conviction," he told BBC Match of the Day. "There was a real improvement and I thought the game was there for the taking in the second half."
Asked about the mood among his players afterwards, Andrews added: "The fact we're looking round the place and there is a tinge of disappointment. Nothing is given to you in this league — you have to go and earn it."
On the penalty award, he was more measured: "I need to be a bit careful. Overall I didn't love the decisions today. In terms of the penalty, I don't have too many qualms about it."
Régis Le Bris, speaking to safc.com, was delighted with his side's performance despite being level at the break.
"I was really happy and pleased with the first half," he said. "I thought we created four clear chances in the first 40 minutes. It was probably one of the best first halves we have had in the Premier League so far.
"We have to be clinical and in those moments today, it wasn't the case. It was a shame to concede this goal because they scored the goal we should've scored. We were good out of possession as well, but in football, if you don't take your chances, the opponent can react and that was the case in the second half."
The Sunderland manager praised his side's response after falling behind.
"The lads stayed connected and felt it was possible to win something here even after the 1-1. It was good for the lads and the substitutes. They gave this extra drive that we need."
He also backed his penalty taker despite a difficult previous campaign.
"It was tough last season for Enzo, but he has learned. I wasn't worried about the penalty because he is a really good footballer."