Chalmers, who closed with a 2-under 69, found the fairway off the tee on the playoff hole, No. 18. Ellis closed with a 5-under 66, but made a mess of his playoff hole.
Ellis' tee ball came to rest in the left rough and he then knocked his second into a greenside bunker. Chalmers dropped his second at Hillcrest Country Club on the putting surface some 25 feet from the cup.
As Chalmers lined up his putt, Ellis blasted out of the bunker. However, that shot came up short in the rough. He pitched his fourth to 3 feet, but it would not matter.
Chalmers, who led entering the final round, stepped up and drained his putt for his first win on American soil.
'I'm really excited at the moment,' said Chalmers, who earned $117,000 for the win. 'It was a hectic finish to that back nine. I was up and down like a yo-yo, but my putter saved me and I needed it to. Danny gave a great effort here today.'
Nathan Green had a chance to join the playoff, but his birdie putt on the last came up short. He ended at 14-under-par 270 after a closing 69 and tied for third place with Troy Matteson, who posted his 12th top-10 finish this year.
Brian Henninger and Camilo Villegas shared fifth place at 11-under-par 273.
Chalmers got off to a quick start with back-to-back birdies from the par-5 second to move to minus-15. He parred his next six holes.
The Australian birdied the 10th to get to 16 under, two clear of Ellis and Green. Chalmers, however, stumbled to consecutive bogeys from the 12th to slide back to minus-14.
Chalmers recovered one stroke with a birdie on the 17th. He saved par from four feet out on the last despite hitting his tee shot into the right trees.
'I made the birdie on 17 after I got a great read from my caddie and I hit a great putt there,' Chalmers said. 'I knew if I could get it in the fairway at 18, it was just a short shot in there. I thought I'd go with a 3-wood and try that because the driver wasn't working.
'Apparently the 3-wood wasn't working either, as that was the worst tee shot I've hit in a while. I snuck out of jail and got a lucky break, but you need luck sometimes. I feel very fortunate to be the champion.'
Ellis had a solid front nine as he opened with a birdie on the second. He climbed to 13 under with a birdie on the par-4 sixth and another on the seventh.
The 35-year-old made the turn at minus-14 thanks to a birdie on the par-4 ninth. Ellis cooled as he parred his next six holes. He got to 15 under with a birdie on the par-5 16th. He could only par the next two holes to get into the playoff.
Jason Schultz and Cliff Kresge tied for seventh place at 10-under-par 274, while Matt Hendrix and Keith Huber shared ninth at 9-under-par 275. Robert Garrigus, Charley Hoffman, Steven Bowditch and Bubba Watson were one stroke further back at minus-8.
Scott Gump, the 2004 winner, was plus-3 over his final three rounds after sharing the lead after day one. He ended in a tie for 36th at 3-under-par 281. There has never been a repeat winner on the Nationwide Tour.