The Bees provided a sting in the tail with two goals in added on time to deny Town the three points the performance deserved.
Blues had edged the chances in an entertaining first-half that looked likely to end goalless until Kevin Bru opened the scoring seconds before the break with an acrobatic finish after Ryan Fraser had hit the post.
It was the Scotsman who put Blues in command early in the second-half, Freddie Sears the creator and Fraser converting his cross from close range.
It looked enough to see Town to victory but substitute Andre Gray gave Brentford an attacking edge when he came on and he gave the home side a lifeline two minutes into added on time.
With the last kick, James Tarkowski gave the Bees a share of the spoils when he forced the ball in from a corner after Blues looked certain to open the season with a deserved win.
Town kicked off with debutants at full-back in Josh Emmanuel and Jonas Knudsen, with Ainsley Maitland-Niles making his first League appearance for the Club since joining on loan from Arsenal in the summer and Fraser doing the same on the other wing following his loan switch from Bournemouth.
Emmanuel was quickly into the action, his dangerous cross from the right hacked clear by Harlee Dean, then Bees’ midfielder Alan Judge fired wide with the first shot in anger.
It needed the fingertips of ‘keeper David Button to stop Town going ahead on 10 minutes though, Sears cutting inside and seeing his shot pushed inches wide of the far post.
Andy Gogia escaped from Emmanuel before Cole Skuse cleared the danger in a free-flowing start to the season.
Daryl Murphy sent his first shot of the campaign high and wide from a difficult angle while at the other end, Brentford’s focal point of their attack, Philipp Hofmann, headed over from a corner.
Maitland-Niles’ cross was diverted towards goal by Toumani Diagouraga after Konstantin Kerschbaumer gave the ball away but Button was alert to the threat.
Judge sent a free-kick just wide from 30 yards, Tommy Smith came to Town’s rescue with a timely clearance as Jota got in on goal, then Sears threatened again before Button was quickly off his line.
Town were getting joy down the Brentford right and Skuse broke clear but Dean was in the right place to chest the ball to safety, Button saved from Bru then seconds later Bart Bialkowski produced a terrific reaction save to deny Jota’s point blank header.
Sears was carrying a real threat for Town and tested Button again, Maitland-Niles then sending a left footer over the bar, Sears denied yet again by the home ‘keeper from close range after a corner had broken to him six yards out but Town finally got the goal the play had deserved on the stroke of half-time. Fraser thumped a shot against the inside of the post and from the rebound Bru acrobatically volleyed in from Emmanuel’s cross.
Knudsen’s long throw will be a weapon this season and Maitland-Niles almost capitalised on the delivery in the first minute of the second-half, the ball just evading the loanee when he was free 12 yards out.
Town didn’t have to wait long though for a second, Sears beat the offside trap with ease from Bru’s pass and he squared for Fraser to hook the ball into the empty net.
Jota couldn’t convert from Alan McCormack’s cross, Button forced into action again to save from Bru to his right then Sears’ driving run thwarted as he looked to tie up the points.
Gray, on as substitute, had too much on his chip over Bialkowski as Brentford looked for a way back into the game, Bru then testing Button again with a right footer the ‘keeper pushed away at full stretch.
The midfielder became the first Town change moments later, cramp forcing his departure on 68 minutes, Giles Coke on for his debut as the replacement.
Fellow debutant Jonathan Douglas joined him on the field soon after, the midfielder given a standing ovation from the home fans after joining Town from Brentford in midweek.
Bialkowski saved from Jota after Gray had toe-poked him in, then Judge fired over after Gray had set him up but deep into added time Gray gave the home side a foothold and they grabbed it with that dramatic last touch of the game from Tarkowski.