The victory leaves the U.S. atop the Group C standings on three points, while Haiti needs a win or a draw against Guatemala on Tuesday to keep its hope of advancing to the quarterfinals alive.
"I'm happy because we got the result against a great opponent," said Richie Williams, the U.S. head coach. "There were nerves at the start."
Despite the apparent early unease, Ahinga Selemani nearly gave the Americans a lead before sixty seconds had elapsed, but Haiti goalkeeper Cleef Cadet was well positioned at his left post to make a stop.
After Jonel Desire found himself on the end of two valid scoring opportunities for Haiti, Jean Wisner Derival saw a 16th minute effort swept away before it reached the U.S. net.
In the 24th minute, Corey Baird was pulled down in the Haitian box and referee Christophel Stewart pointed to the spot. Two minutes later, Lema calmly stroked his penalty kick to the left of a diving Cadet to give the defending tournament champion a 1-0 lead.
Arcus Carlens nearly leveled for Haiti just prior to the interval, forcing an alert Elijah Martin to head off-the-line at the left post.
Lema netted his second goal in the 73rd minute, finding the back of the net with his left foot from 18-yards off a headed assist by Rubio Rubin, who guided a Baird cross in the direction of the New York Red Bulls youth product.
Selemani got his name on the score sheet four minutes from time. The 17-year-old shrugged off a Haitian defender on the left side near the end line, regained possession after the ball took an unusual spin and placed a shot a right-footed shot past Cadet.
The U.S. is idle until Thursday, when it faces Guatemala.