Henry returns to send Gunners through
The former France international, 34, left the Gunners in June 2007 as the club's record goalscorer and returned last week on a two-month loan from New York Red Bulls.
He could hardly have made a more impressive comeback, showing his colleagues how to finish after Leeds, currently eighth in the Championship, held out stubbornly until the 79th minute, 11 minutes after Henry had come on to a rapturous welcome from the home fans.
It was his 227th Gunners goal and Arsenal's reward is a fourth-round tie at home to Aston Villa. Henry will probably start that one, as well as be involved in UEFA Champions League fixtures.
The 1998 FIFA World Cup™ winner settled a game that was a repeat of the draw for the same round last season, which saw Leeds come within a handful of seconds from winning at the Emirates.
After the match he told ESPN: "It's kind of weird, I only came back from holiday in Mexico 15 days ago, I never thought I was going to be playing for Arsenal again and then score the winner.
"The feeling I had when I scored was amazing, I've rejoined the club as a fan, I just wanted to come and help the team, and then I end up as man of the match - it wasn't my plan to be a hero. I'll always remember tonight. When it comes to Arsenal, something happens to me."
Arsene Wenger had allowed captain and top scorer Robin van Persie to take a holiday, with Marouane Chamakh taking his place up front before departing for the CAF Africa Cup of Nations.
Otherwise it was close to being as strong a starting line-up as a series of injuries to defenders would allow, with Wojciech Szczesny, Aaron Ramsey, Alex Song, captain for the night Mikel Arteta, and Andrey Arshavin all included.
Henry, as expected, began on the bench, as did Theo Walcott, the inheritor of the number 14 shirt the Frenchman left behind four-and-a-half years ago when he decamped to Barcelona.
Hosts start well
An early error by Leeds defender Aidan White allowed Chamakh to give Arshavin a clear sight of goal but the Russian blasted his shot well over Andy Lonergan's crossbar.
Sebastien Squillaci headed just wide from an Arteta free-kick in the 15th minute before Aaron Ramsey fired over on the run from the next attack. Ramsey twice went close in the next five minutes, with Arteta also sending a low drive just wide.
Leeds replied with a shot from Mika Vayrynen that bounced back off Laurent Koscielny to Luciano Becchio, who fired over to the obvious chagrin of Simon Grayson, the Leeds manager.
Arsenal's defensive problems increased when Francis Coquelin, playing at right-back, pulled up with a hamstring injury and had to be replaced by 18-year-old Nico Yennaris in the 33rd minute.
The second half began with a run from Alex Oxlade-Chamberlain and a shot that went wide, and Arshavin failed to hit the target with a first-time effort from Ignasi Miquel's diagonal ball forwards.
Still Arsenal came forward in search of unlocking a packed Leeds defence. Lonergan was required to dive to his left to keep out Arteta's shot after Chamakh had set him up, before Arshavin fired across goal and just wide.
Sub Henry strikes
Lonergan was required again to beat away Oxlade-Chamberlain's drive from distance as Henry prepared to make his 68th-minute entrance, along with Walcott. Henry's magic moment arrived in the 79th minute and it was a finish typical of the player in his previous prime.
Song played the ball forward, Henry timed his run behind the Leeds back line perfectly and slotted across Lonergan before embarking on a lively celebration in front of his manager. Leeds did not give up and Arsenal required a smart save from Szczesny in stoppage time to deny substitute Mikael Forssell.