Clark's homer followed a game-winning shot by Eric Byrnes the previous night, and is the Diamondbacks' fifth game-winner this season. Arizona has won seven straight to move within one game of the Los Angeles Dodgers in the NL West.
Edgar Gonzalez (4-2), the Diamondbacks' fifth pitcher, worked the 10th and 11th for the win.
Wilfredo Ledezma (0-2) entered the game in the 11th and gave up the homer to Clark, the only batter he faced. The Braves have lost three straight and remain four games behind the New York Mets in the NL East.
Arizona was one out away from securing the victory in the ninth, but a mental error by Jose Valverde helped send the game to extra innings.
Valverde got two quick outs before walking pinch hitter Yunel Escobar. Valverde circled to the right of the mound with his back to first base bag as Escobar trotted down the line. Escobar paused at first long enough to touch the bag, and when Valverde crouched at the side of the mound, Escobar broke for uncovered second base.
The next hitter, Willie Harris, fisted a bloop single into shallow center, tying the game at 7.
Arizona scored the first seven runs over the first four innings - including a six-run second - only to watch the Braves rally.
Diamondbacks starter Yusmeiro Petit didn't allow a hit until Jeff Francoeur's single to left in the fifth, but faced only nine more hitters.
Kelly Johnson tripled, scoring Francouer, and Julio Franco had an RBI single to make it 7-2.
The Braves added four more in the sixth. Edgar Renteria led off with a single and Andruw Jones and Brian McCann hit consecutive one-out ground-rule doubles to cut the lead to 7-4. Petit retired Francoeur for the second out of the inning before giving way to Juan Cruz.
Cruz hit Johnson with a pitch, then surrendered Franco's two-run double into the right-field corner, pulling Atlanta within a run. Cruz retired pinch hitter Matt Diaz on a broken-bat fly ball to center to end the threat.
Chris Snyder started the Diamondbacks' six-run second against Jo-Jo Reyes (0-2) with a one-out double. Snyder scored on Drew's home run to right, his seventh.
One out later, Petit singled and Young walked. Orlando Hudson doubled to left, scoring Petit and Young to make it 4-0. Eric Byrnes followed with a towering 418-foot home run to left, his 16th, to cap the inning.
Young's two-out solo homer in the fourth, his 17th, gave Arizona a seven-run lead.
Reyes, making his fourth start since he was recalled from Triple-A Richmond July 7, allowed seven runs and seven hits over four innings with a walk and two strikeouts.
Notes: Byrnes homered in the bottom of the ninth Thursday to beat the Florida Marlins. ... Clark has two career game-winning homers. His last was September 18, 1998, against Minnesota as a member of the Detroit Tigers. ... McCann threw out all three would-be Diamondbacks base stealers. ... At a news conference earlier Friday, the Diamondbacks announced left-hander Randy Johnson will undergo season-ending back surgery. The 43-year-old left-hander was 4-3 with a 3.81 ERA in 10 starts. ... Arizona traded OF Scott Hairston to the Padres for minor-league RHP Leo Rosales and recalled OF Carlos Quentin from Triple-A Tucson. Hairston hit .222 in 76 games for the Diamondbacks. Quentin, who was hitting .210 when he was sent down on July 6, was in the starting lineup Friday and went 0-for-3.
