Johnson hit two of Arizona's four homers, helping the Diamondbacks rally for a 5-3 victory over the San Diego Padres on Wednesday night.
Johnson connected on Kevin Correia's second pitch of the game and added a tying two-run drive in the fifth. His homers traveled an estimated 852 feet combined.
"That happens sometimes when you're not thinking about trying to hit a homer," he said.
Johnson went 3 for 3 with a walk in his first game of the season on top of the lineup. The second baseman, who signed a free-agent deal with Arizona in the offseason, batted eighth in the first two games of the season and went 0 for 7 with a walk.
Johnson batted leadoff in 145 games over four seasons with Atlanta but said he doesn't care where he bats in Arizona's order.
Scott Hairston hit a three-run homer in the second for San Diego.
Ian Kennedy struck out eight over five innings in his first start for Arizona, yielding six hits.
"I felt like he did a heck of a job of keeping us in the game and regaining his composure," Diamondbacks manager A.J. Hinch said.
"We couldn't really muster anything against their bullpen," San Diego manager Bud Black said. "After the second inning, Kennedy shut us down, too."
Correia (0-1) allowed four runs and seven hits in six innings.
"Everything felt great, I just made a couple of mistakes," Correia said. "I felt that I gave that game away. One good pitch in any of those situations and we still have the lead."
Padres LHP Joe Thatcher, who is on the 15-day disabled list with a sore shoulder, threw on the side. Manager Bud Black said he felt good and threw the ball fine. Thatcher will be re-evaluated for a possible rehabilitation assignment. ... Johnson's first homer was his sixth career leadoff shot and the 40th in club history.