A handball in the box gave the Bairns striker the chance to snatch a point from Clydeside and he seized it -- but it still wasn't enough to get an angry travelling support off the Bairns' backs.
Protest chants rang out throughout the game, but grew in volume and venom after the Sons took the lead on the half-hour and anger threatened to spill over at the interval.
McManus' goal from the spot denied Jim Duffy's side a move up the SPFL League One table - and Falkirk remain fourth, three points off new leaders Airdrie.
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For half an hour the play from the Bairns was progressive and positive in their tweaked new-look with Gary Miller screening the defence and both Aidan Connolly and Charlie Telfer restored to the midfield.
LEAGUE TABLE: Ladbrokes SPFL League One
But unable to find a goal, the Bairns were hit with a sucker-punch on the half-hour and crumbled. The support for the team dwindled quickly too and the scenes at half-time turned poisonous as a section of the away fans gathered as close as they could to the tunnel to barrack the Bairns off the field, spilling over stewards and barriers.
The loudest reaction was saved for Ray McKinnon as he crossed the field and he was met with a chorus of boos as he returned to the dugout after the break. His second half substitutions, two made at once, were also criticised and met with a chant of "You don't know what you're doing." Just three weeks ago the manager was slammed for NOT making a change early enough.
By the end, the attitude from the away section of the single stand at Dumbarton had turned to silence - either indifference indifference or anger, or into protest against the management of the team, and the club.
But at the start it was different with McKinnon changing the team, and Louis Longridge went close twice after six minutes, but Conor Brennan beat out his low drive then he watched Longridge's effort from Gregor Buchanan's lay-off ciurve just outside the post.
Michael Doyle was cautioned for taking the fight to PJ Crossan after a quarter of an hour. The joker in the Bairns pack taking a serious approach to the current predicament and his aggression was fortunate not to go ovr the score and he remained on the pitch though like all bar Buchanan across the Bairns baclkline he had a tough afternoon where little paid off after the Sons took the lead.
Yet again, the Bairns just couldn't find the net to match their early pressure and Morgaro Gomis fired over after Telfer and Connolly couldn't fashion an opening for themselves, then Declan McManus fired a strike off the back of Ryan McGeever close in.
HOW IT HAPPENED: Matchday live blog
Prolifigacy came back to haunt them and on only their third real attack, Dumbarton took the lead and turned the Bairns support toxic. Isaac Layne poked in at close range after a low drilled cross from the left side and both Ray McKinnon and the defender on that side of the park, Paul Dixon, came in for direct criticism.
So too did the club board and what followed was an ugly reaction to the half-time whistle near the mouth of the tunnel after a 15 minute period where Falkirk played punch-drunk by Layne's sucker-punch and little came off for the Bairns in a truly awful spell.
The reaction to the angry scenes from the fans was not enough to turn the game in the Bairns' favour. Declan McManus couldn't turn in a Buchanan header in what has become the Bairns most frequent route to goal this season and the forward was hauled down on the edge of the box for a chance which Aidan Connolly fired just over, via a Dumbarton deflection in the defensive wall.
But even after making two changes, David McMillan on for the bright Louis Longridge and Ross MacLean for Charlie Telfer, the tide did not turn in the stands nor on the pitch until the last two minutes
The Sons even had the ball in the net again with a carbon copy of the opener, but Bairns blushes were spared by the linesman's flag. The reprieve was finally seized by Declan McManus from the penalty spot with two minutes to go after a strike from McMillan struck an arm and the Bairns bombarded the goal but it was too late and they left with a single point and the jeers of the visiting contingent ringing in their ears.