After doubling to center in his third at-bat for his first big league hit, he drove in the go-ahead run with a sacrifice fly to left field in the ninth.
"He had an outstanding game. There was nothing I didn't know," manager Davey Johnson said about Harper. "He had quality at-bats. He made one hell of a throw, running hard."
For reasons, he couldn't explain, Harper said he wasn't nervous before the game.
"I didn't have butterflies at all, really," he said. "It was the first time I didn't get butterflies. I was talking to [Adam] LaRoche before the game. I told him, 'Hey, I'm really calm right now.' I wasn't upbeat or anything like that. I was pretty calm. I was trying to look for my pitch and get into some good counts."
Harper grounded out in his first at-bat against right-hander Chad Billingsley, in the second inning. With the count two balls and one strike, Harper grounded out to the pitcher to end the top of the frame.
Harper came to bat again with a man on first and one out in the top of the fifth, and worked the count to 2-1 before flying out to left fielder Tony Gwynn Jr.
Two innings later, Harper found himself in a full count in his third time facing Billingsley, and hit a double over the head of Dodgers center fielder Matt Kemp.
Harper's first fielding opportunity was a success, as he caught Jerry Hairston Jr.'s fly ball to left for the second out in the second.
Harper came close to throwing out a runner in the seventh inning. With runners on first and second and one out, A.J. Ellis singled to left field. Harper made a perfect throw to the plate, and it appeared that Hairston was going to be nailed at the plate, but catcher Wilson Ramos dropped the ball, and that allowed Hairston to score the tying run.
The replay showed that Hairston swiped the ball out of Ramos' glove. Johnson argued the call, to no avail.
"[Hairston] smacked the ball. He is out. We couldn't get any help [from the umpires]," Johnson said. "They obviously didn't see it."
Harper was anticipating a bang-bang play at the plate.
"I was trying to make a good throw, and have a good opportunity to try and throw him out. I did the best I could," Harper said.
Although he did well in his first game, Harper wished the Nationals won the game.
"I wish we would have gotten the W, of course." Harper said. "We played a great team today. Billingsley threw a great game. We fought until the end. That's not the way you want to start off your career. I just wish we got a W."
Harper is on Washington's 25-man roster because third baseman Ryan Zimmerman was placed on the 15-day disabled list due to inflammation in his right shoulder. Harper will be the everyday left fielder, and batted seventh in the batting order in his debut.