The Pirates had lost 10 straight to the Cubs and were 0-7 against them this year before McLouth came through with the big hit.
Freddy Sanchez beat shortstop Ryan Theriot's throw to first on a grounder up the middle with one out before McLouth's drive off Marmol (1-1) to the basket above the right-field wall. Pittsburgh won for the ninth time in 12 games.
Matt Capps gave up a two-out RBI single to Ryan Theriot after Soriano doubled for his fifth hit of the game, but retired Derrek Lee on a fly to right for his 10th save.
Damaso Marte (3-0) pitched a perfect eighth.
Soriano has 20 hits in his last 37 at-bats, raising his average to .295 after a brutal start.
He tied a career high by leading off his fifth straight game with a hit when he lined a single to center and added an RBI single during a three-run second that gave the Cubs a 3-1 lead.
His solo shot to left off Zach Duke in the fourth made it 4-2, and he greeted reliever Tyler Yates with another one that tied it at 5 in the seventh. That gave him seven homers in the week and 10 for the season. He became the first Cubs player with back-to-back multihomer games since Aramis Ramirez at Washington on July 21-22, 2006.
Cubs starter Carlos Zambrano was in line for his second loss of the season before Soriano's second homer. After going 5-0 in his previous six starts he simply didn't have it. He left with none out in the fifth and the game tied at 4 after lasting at least six innings in his first nine outings.
Duke led off with a walk, and Sanchez and McLouth singled to make it 4-3. Zambrano then walked Jason Bay to load the bases before Adam LaRoche chased him with an RBI single off his glove that tied it. Nady then made it 5-4 with a sacrifice fly to foul territory in right off Michael Wuertz, before Doug Mientkiewicz lined a double play to first baseman Derrek Lee.
It was a frustrating day for Zambrano, who gave up a season-high five runs on seven hits. Besides struggling on the mound, he popped out while trying to lay down a bunt in the second and broke his bat over his left leg after striking out in the fourth.
Duke, who won his previous two starts, gave up four runs and eight hits in 6 1-3 innings.
He left after Nady made a diving catch near the right-field warning track to rob pinch hitter Ronny Cedeno and Soriano went deep off Yates.
