The 34-year-old has rejoined his old club on a short-term loan deal from the New York Red Bulls ahead of the start of the new Major League Soccer season.
Henry became Arsenal's record goalscorer during eight trophy-laden seasons with the north London club before leaving to join Barcelona in 2007 and last month was honoured with a bronze statue outside Emirates Stadium.
After watching the forward train with the first-team squad in recent weeks, Wenger was convinced Henry remains a champion.
"I watched the [MLS] play-off against LA Galaxy. That is when I saw Thierry and thought he has still got the pride and desire to win," said Wenger.
"It was still there because he was so angry and I didn't expect that from him. I thought he would be a bit cooler.
"That was the moment when I saw the fire was still there.
"I saw his desire to go and get the ball back. You could see the desire to fight and the pride to win is there.
"You could see that the champions never lose that, you saw that with David Beckham and Thierry has that as well."
Wenger added: "It is how he can cope physically every day with the training and how much he wants it as well.
"I was quickly convinced, but I wasn't sure he wanted the challenge as well."
Henry will be available for Monday night's FA Cup third-round tie against Leeds at Emirates Stadium, then the upcoming Barclays Premier League encounter against Manchester United at Emirates Stadium, as well as the first leg of their Champions League last-16 clash at AC Milan, while his deal could yet also include the clash with Tottenham on February 26.
Wenger revealed he wanted to see first hand whether Henry could still cut it in the top flight and also make a difference to what is still a young squad short on experienced winners.
"It took me two to three weeks to make the decision because I had to get used to the idea to see what kind of impact he would have with the team, how he was physically and how motivated he is to come here every day and work hard," said Wenger.
"I don't know how it is in the United States, but I know how high the demands are physically in the Premier League.
"They are absolutely exceptional and the highest in the world so you have to see how much he can cope with that."
Wenger rejected criticism the move for Henry was in fact a step backwards and an admission his current transfer policy had failed.
"I wanted Thierry to stay, but he said we have a young team who will be good in three or four years, but I cannot wait, I am 30 and need to go somewhere where I have a chance to win, which was completely logical," the Arsenal manager said.
Wenger accepts the FA Cup has now "added importance" after Arsenal's exit from the Carling Cup to Manchester City denied them another shot at ending what will be a seven-year trophy drought.
"Last year we played in every single competition with top commitment," the Arsenal manager said.
"We went to the final of the Carling Cup, which we lost, then went out [of the Champions League] a few days later in Barcelona and then lost to Manchester United in the FA Cup.
"In one week, it was all away, but we always try to do well in the FA Cup."