English winger Sam scored in the eighth minute with a delightful chip over DC keeper Bill Hamid from an angle just moments after Cahill had been hacked down in the area before United levelled in the 36th minute through Nick DeLeon.
Australian midfielder Cahill edged the Red Bulls back in front two minutes later with a customary header from a floated Jonny Steele cross and the performance of New York's two goalscorers was not lost on head coach Mike Petke.
"I thought Lloyd did phenomenal. I thought we actually didn't get him enough of the ball in the first half, but when he got it he beat his man to provide quality crosses," said Petke.
"And Tim, you know, we thought it was a good game to have Tim closer to the goal instead of making 30, 40 yard runs into the box as a late runner and tiring himself out. But it was a great cross by Lloyd and a great finish by Tim for sure."