The Reds snapped a three-game losing streak with the win, while the Cubs had won six of their last eight games, which included a three-game victory stretch.
In his final start of the 2011 season due to an innings limit, Leake (12-9) allowed six hits while walking two and striking out two.
"It was nice to finish on a good note," Leake said. "I really didn't have too many personal goals. My main goal was to stay healthy throughout the year and then finish the year. Fortunately, I was able to do it."
Leake threw 138 1/3 innings in 2010 and was shut down on August 24. This season, Leake tossed 167 2/3 frames and lasted until the middle of September.
Ryan Dempster (10-12) picked up his fourth loss in his last seven decisions despite throwing a season-high 128 pitches in seven innings. He allowed two runs on six hits and six walks. He struck out eight.
"Sometimes it's not all about velocity and overpowering stuff," Cubs manager Mike Quade said. "Both those guys pitched very well."
Dempster has not won a start since August 11 against Washington.
After the aforementioned Phillips lead-off homer, Cincinnati tacked on another run in the fifth on a base hit to center field by Renteria. He brought home Leake, who reached on a fielder's choice.
Francisco Cordero notched his 33rd save of the year with a 1-2-3 ninth.
Leake leads all Reds starters with 12 wins, he is 4-1 in his career against the Cubs...Dempster threw over 100 pitches for the 19th time this season...Castro's 13-game hitting streak was snapped...Phillips has hit safely in 24-of-26 games since hitting lead-off...Cordero has saved 16-of-16 games since the All-Star break.