As well as 19-year-old McKennie, caretaker coach Dave Sarachan also gave 18-year-old Tyler Adams his first outing in the match in Leiria -- the first time since 1988 that two teenagers have started a game for the U.S.
Portugal, who unlike their opponents safely qualified for the World Cup, also fielded an experimental side, resting players like Rui Patricio, Bernardo Silva, Ricardo Quaresma, Cristiano Ronaldo and Joao Mario.
The U.S. went ahead in the 21st minute after the ball was played out to C.J. Sapong on the left wing and he slipped it to midfielder McKennie, who outfoxed his marker before scoring with a classy finish.
The European champions were gifted an equaliser 10 minutes later when U.S. goalkeeper Ethan Horvath allowed a harmless Vitorino Antunes cross to slip through his legs and into the net.
Both teams had chances after the break, with Goncalo Paciencia hitting the bar for Portugal, before the game petered out into a draw.
Portugal defender Pepe limped off with a foot injury in the 10th minute but it did not appear to be serious.
Fernando Santos, the Portugal coach, was not satisfied with his team's display.
"Our incapability of keeping the ball was a negative aspect. Our opponents were aggressive and didn't let us think," he said.
"In the first half, we didn't have enough of the ball and we kept losing it, we were too rushed."
"We managed to knock it around more in the second half but not as fluidly as normal. We never managed to totally dominate the game."
(Writing by Brian Homewood, editing by Nick Mulvenney)
