Arsenal dominated Wednesday's match at Highbury and was rewarded in the 41st minute with Toure's first goal in 14 months.
"I believe it is a good result," Arsenal manager Arsene Wenger said. "But we will know after the second game."
"I don't think we played particularly well," Villarreal manager Manuel Pellegrini said. "But it is a scoreline that's not too bad.
Kolo Toure (right) challenges Jose Mari in a 1-0 win Wednesday at Highbury. (F. Bustamante/Associated Press)
"We can get a good result in Spain [in next Tuesday's second leg at El Madrigal] and go through."
After Thierry Henry's corner kick was deflected out of danger, he regained the ball and passed to Alexander Hleb, who chipped the ball to Toure.
The defender simply slipped it by Villarreal goalkeeper Mariano Barbosa to put the Gunners ahead 1-0.
Arsenal keeper Jens Lehmann preserved the margin late in the half, smothering a dangerous free-kick from Juan Ramon Riquelme.
"We didn't play well and that's why we lost," Villarreal striker Diego Forlan Forlan said. "We need to score and not let them score.
"And it's not going to be very easy. We tried to do our best and we could not get forward."
Henry had a goal disallowed in the 12th minute on a questionable offside call.
"We have nothing so far," Henry said. "We are 1-0 up, but they really made it difficult for us."
"There was a goal more in the game for us," Wenger said. "We had some very good situations we didn't take advantage of."
Play was halted briefly as a squirrel scampered on the pitch, prompting Wenger to quip: "He was quite fast and dribbling."
Arsenal is on target to reach its first Champions League final, to be played May 17 in Saint-Denis, France.
In the other semifinal, FC Barcelona prevailed 1-0 over host AC Milan in Tuesday's first leg at the San Siro.
Both return legs of the home-and-home, total-goal semis will be contested next week.
"The next six days really define our season," Wenger said. "We are confident we can do it."
with files from Associated Press
