Harper was hitless in his previous 12 at-bats before the home run, which landed in the grassy batter's eye in straightaway center field. Harper was hit by a Julio Teheran fastball in his next at-bat, yelling and pointing at the Braves right-hander as both benches and bullpens emptied onto the field.
With a 2-1 loss to the Braves on Tuesday, the Nationals fell to 28-7 in games when Harper hits a home run.
"I'm going to go out there and play every single game like it's my last," Harper said after the game. "I play this game hard every day and that's what I'm gonna do and hopefully everybody jumps on that train."
The Nationals' top pick in the 2010 First-Year Player Draft, Harper hit 22 home runs during his rookie season in 2012, the most in a single season by a teenager since Conigliaro's 24 in 1964 for the Red Sox.
Harper, who will turn 21 on Oct. 16, also was the youngest All-Star starter in National League history this season. He finished second to Oakland's Yoenis Cespedes in the Home Run Derby with 24 home runs.
Tom Schad is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.