Townsend scored the crucial third goal in the 4-1 win over Montenegro which means England maintained top spot in Group H ahead of Tuesday's must-win clash with Poland.
"Andros Townsend was the difference," Cahill said.
"I thought he was phenomenal down the wing. Every time he got the ball he was positive.
"It was good to see him get the goal as well as set one up. He was brilliant.
"The third goal was a fantastic finish and when that went in the game was over."
Ukraine beat Poland to keep up the heat on England and face San Marino on Tuesday knowing they can take advantage of any slip up from Roy Hodgson's men.
Chelsea defender Cahill felt the tension in a goalless first half before a dominant second saw England take their chances.
Cahill said: "You could feel the atmosphere, feel the pressure in the game.
"It was a good win. We've got one more game to seal it now.
"We need to go out with the same mentality, try to be positive and get some goals."
England are, at the very least, in the play-offs, but automatic qualification is the aim.
Cahill added: "We don't want to think of that option. We've got one game at home against a good side.
"If we go about it in the same manner I think we'll be okay."
England's players knew their fate would be determined in the final game.
Cahill's central defensive partner Phil Jagielka said: "We needed two wins from two. It's what we pretty much expected. One down, hopefully one to go.
"We've got half the job done. We've got the other half to try and complete on Tuesday."