Henderson's late winner gave Forest manager Billy Davies something to enjoy from the directors' box as he served a one-match touchline ban.
The Reds boss was absent from the sideline as a result of an angry confrontation with referee Craig Pawson at Wigan but saw his side show their character to claim a valuable three points against the struggling Tykes who pushed them all the way.
A Chris O'Grady penalty had cancelled out Simon Cox's opening goal and a storming free-kick from sub Tomasz Cywka pulled Barnsley level for a second time, following a Kelvin Etuhu own goal, before Henderson had the last word.
The first clear cut opportunity came in the 10th minute, when Forest were denied by the width of the post. Andy Reid and Radoslaw Majewski combined well to send Cox through on goal but the striker's placed shot bounced away off the inside of the woodwork.
Pollitt made an outstanding save to deny Jamie Mackie after Cox had sent him racing clear but Forest finally took the lead in the 18th minute despite the best efforts of Pollitt, who again pushed away a Mackie shot but saw the alert Cox pounce to slot home the rebound before he could regain his feet.
The home side's advantage was to last less than two minutes however, with Guy Moussi punished for a clumsy, robust challenge on David Perkins in the penalty area. O'Grady finished confidently from the spot, firing high to Karl Darlow's right to make it 1-1.
A spectacular Reid free-kick took a deflection to steer it onto the bar and, when Cox attempted to drive home the loose ball for the second time, Pollitt this time managed to pounce on his shot on the line.
Pollitt was still sprightly at 41, this time plunging quickly to his left to push away a Majewski drive that seemed bound for the net. But the keeper could do nothing in first-half injury time as Forest took the lead for the second time.
A Reid delivery from a free-kick wide on the right was again integral as he whipped in a left-footed cross that had the Barnsley defence in a panic - and saw Etuhu steer the ball into his own net.
But after Moussi had fouled Dawson, Barnsley drew level again in the 69th minute in breath-taking fashion, with sub Cywka hitting a superb free-kick that was still rising as it found the corner of the net from 30 yards.
There was more drama to come however. One Forest substitute, Jamie Paterson, was denied by a post before another, Djamel Abdoun, forced a scrambling save from Pollitt as he sent a cross bending towards the far post.
And, from the resulting corner, Forest's third substitute, Henderson, rose to thump home a powerful header from Reid's cross and seal the points.