Buried on the bench in late June and traded to Florida in July before a last-minute snag quashed the deal, Jones has gone from rags to riches like no other Cub.
Jones' two-run, sixth-inning homer off Braden Looper halted the Cardinals' five-game win streak and helped vault the Cubs into sole possession of first place.
"I figured as bad as I was in the first half, I didn't have anything to lose in the second half," Jones said.
Rich Hill earned his first win since July 16 with seven strong innings, allowing only three hits, including a solo home run by Albert Pujols in the sixth. Bob Howry retired Pujols with two men on to end the eighth and Ryan Dempster nailed down his 18th save (and first since July 24th) with a perfect ninth.
Are the Cubs finally out of their funk?
"It's hard to tell," Howry said. "I don't think you can tell after a couple of days. It's something you've got to let play out over the next week, see how we finish off this series. Playing the way we were, we'd be going home at the end of the season. We have to turn it around again.
"But we know we're capable of it. You saw we played bad for two months, and we played really well for two months. We've got to get back and get going again."
For the first six innings, the Cubs and Cardinals traded volleys in tit-for-tat fashion, including a batter's interference call on Scott Rolen and then a no-call on Derrek Lee when he appeared to interfere with Cardinals catcher Yadier Molina.
Jones received a standing ovation in the fifth after a great catch in the vines saved a run and ended the inning. Then Cardinals left fielder Ryan Ludwick made a diving catch to rob Matt Murton in the bottom of the inning.
Hill's only mistake came in the sixth, after Pujols, on a 2-2 pitch, hit a sky-high popup that landed in the first row of seats behind the tarp down the left-field line. Shortstop Ryan Theriot pounded his fist into the tarp, knowing Pujols would get another chance.
It came moments later when Hill threw a 91-m.p.h. fastball that Pujols slammed into the left-field bleachers. Hill stared at the ground with his hands on his knees as Pujols rounded the bases.
But after Theriot's one-out single in the bottom of the sixth, Jones cranked a two-run shot to left on a 3-2 pitch, pumping his fist as he circled the bases to give the Cubs a 2-1 lead.
"He's really been carrying us lately," Theriot said. "That's what a team is all about. You've got to lean on guys sometimes, and without him I don't know where we'd be right now."
Pujols re-emerged in the eighth with two on and two outs, following Howry's walk to pinch-hitter Rick Ankiel. After falling behind 2-0, Howry induced Pujols to pop up to short right, where Mike Fontenot called off Jones to make the inning-ending catch.
Dempster breezed through the ninth on nine pitches, giving the Cubs first blood in the four-game series.
"I'm just glad Bobby walked Ankiel so I didn't have to pitch to Pujols," Dempster said with a smile.
"Yeah, I was just looking out for him," Howry joked.
But mostly the players were looking out for Jones, who has experienced the agony and the ecstasy of life as a Cub in a few short weeks. He's hitting .360 since the All-Star break, after nearly becoming an ex-Cub.
Jones said the satisfaction he felt was for his teammates, not himself.
"Just for the guys," he said. "For the team, and for Rich pitching his butt off. Guys were making plays. We're getting down to the nitty-gritty now."