Nick O'Hern shot a 70 to take second place alone at 10-under-par 278. Graeme McDowell, who carried a three-shot lead into the final round, struggled to a 77 and tied for third with Raphael Jacquelin at 9-under-par 279.
Harrington, who won the season-opening event in Hong Kong last December, had only one top-10 finish, a share of second at the Irish Open, in his last seven events on the European Tour. He missed a cut in his previous start at the BMW International Open, but the Irishman arrived at Gut Lrchenhof with the intention of leaving his shaky play behind him on the eve of the Ryder Cup.
'It's nice to come back and show some form,' said Harrington. 'I was surprised it turned around so quickly, but I'm very happy about it.'
Harrington held the first-round lead but struggled to a 75 on Friday. He fired a 64 on Saturday to get within striking distance of McDowell, but things looked to be falling apart early on Sunday after Harrington bogeyed each of his first two holes.
McDowell, who was also seeking his second win of the season, stuck his approach to 3 feet at the first and converted the short birdie putt to reach 15 under.
Harrington recovered from his early mistakes with a birdie at the third to pull within five of McDowell, but the momentum was about to shift. McDowell missed a 6-foot putt for a bogey at the fifth and Harrington sank a 5-footer for a birdie at the par-5 seventh to get within three strokes of his playing partner.
After a par at the eighth, McDowell headed to the difficult par-4 ninth for what would be the beginning of the end for the Ulsterman. McDowell sent his second shot in a greenside bunker but was left with a difficult lie. He hit his third shot clean and watched as it carried over the green and rolled into the water.
McDowell took a drop and walked away with a double bogey. Meanwhile, Harrington, who had found the rough off the tee, knocked his approach to 14 feet and ran home the putt to join McDowell atop the leaderboard at 12 under.
McDowell's problems continued at the par-3 11th when he sent his tee shot into a bunker en route to a bogey.
Now in the lead on his own, Harrington birdied the par-5 13th but McDowell matched his with a birdie of his own. McDowell then bogeyed the 14th to fall two shots off the pace.
Harrington extended his advantage with a birdie at the 15th and cruised to the title despite a bogey at the penultimate hole.
'I was just trying to get the last couple of holes out of the way,' said Harrington. 'It's a tough golf course to lead on.'
McDowell wasn't out of the woods yet and the 25-year-old double bogeyed the par-3 16th to fall back into a tie for third.
O'Hern, still searching for his maiden European Tour title, collected four birdies and two bogeys for his second runner-up finish of the season.
Germany's Alex Cejka finished alone in fifth place at 8-under-par 280. Paul Casey capped off his round with a double bogey and a bogey over his last two holes to join Nick Dougherty and Richard Green in a tie for sixth place at 5-under-par 283.