The Australian international headed home Juninho's floating free-kick in the 91st minute as the Red Bulls extended their unbeaten streak to six game to go top of the Eastern Conference table.
The former Everton ace admitted that he and teammate Henry spoke about blocking the run of Galaxy centre-half Omar Gonzalez in order to win the ball in the air.
"I asked Thierry to put the block on Gonzalez," Cahill told reporters after the match.
"He's a big guy, heading everything away, and I knew where Juni was going to put it, I knew it was going to be dangerous. It was just [about] getting contact."