Along with his production at the plate, Upton made the crowd at Petco roar in the eighth inning, as the left fielder came up with a big-time catch.
With relief pitcher Joaquin Benoit on the mound, Pedro Ciriaco hit the ball hard to left, forcing Upton to run backward and leap into the wall to make the grab for the first out of the inning.
"Upton's play was incredible," interim manager Pat Murphy said. "Maybe a game-changer."
Benoit was able to get out of the inning and made sure to greet Upton with a hug before heading back into the dugout.
"Right before I got to the [warning] track, I checked and saw where I was. Most of the time, you can gauge on the ball, so I just told myself, 'Go ahead and catch it, you know where the wall is at,'" Upton said. "At that point in the game, you got to make the catch, you can't give yourself up."
Despite playing against his former team, Upton said his mentality didn't change when entering Monday's game. Instead, being around his former teammates is an enjoyable experience for him.
"You are playing against guys that you went into the clubhouse with the year before -- it's fun," Upton said.
The Padres' 2015 All-Star has been hitting well in the month of August, batting.286 with an on-base percentage of .410.
Although Atlanta was his home last year, the chants of Upton's name were loud on Monday during his final at-bat in the ninth inning. Upton said that San Diego has felt like home this season.
"Our fans have been great all year long, tonight was a Monday night and we had a really good crowd," Upton said. "They are into this team, and we are trying to go out and produce for them."