In the final minute of an entertaining encounter, United defender Darren Dods pulled down Giovani, allowing the former Arsenal player, on as a second-half substitute, the opportunity to score from the spot.
United’s debutant keeper Grzegorz Szamotulski saved the first effort but Henry followed up to knock the ball into the net.
It could have been different if Barry Robson’s 82nd-minute header had not been chalked off for a push on Barca defender Eric Abidal and if Lee Wilkie’s last-minute header had not been headed off the line by Julian Belletti, but in the end the classier side won through.
Meanwhile, Albert Luque proved the unlikely hero as Sam Allardyce tasted victory at St James’ Park at the first attempt — in a pre-season friendly against Celtic.
The forgotten Spaniard, who had been promised a chance to prove himself all over again, grabbed his opportunity with a first-half double to leave Scottish champions Celtic reeling.
He and Obafemi Martins combined to hand their side a 3-0 lead at the break as the frailty of the visitors’ defence proved more costly than that of their hosts.
The Magpies did not have it all their own way by any means as the Hoops played some neat football, but while Luque and Martins were decisive in front of goal, it took Celtic 72 minutes to find a way past former trainee Shay Given as Scott Brown claimed a consolation strike.
And despite Celtic finished strongly as Allardyce continued to make changes — reserve keeper Steve Harper played the last 11 minutes as a striker — Magpies forward James Milner wrapped up the win with an excellent last-minute strike.
Nottingham Forest slipped to a 2-1 defeat at Dunfermline, courtesy of a late goal from the Scottish side.
The home team took the lead in the first half through Mark Burchill’s close-range header — but Forest were almost immediately back on terms after Kris Commons’ deflected shot fell kindly for Emile Sinclair to tap in.
The Pars clinched the win with a minute left when Owen Morrison cut in from the wing to rifle home.