He shot 72 and finished at 13 under. Johnson said. "I don't know why it hasn't happened but the US Open is a tough, tough place to go and win". Putnam's solitary birdie of the round came two holes later to narrow the gap to two shots, but he wouldn't come any closer than that for the remainder of the round.
Johnson won his first major title at the 2016 U.S. Open and was a runaway world number one heading into last April's Masters, only to bruise his back when he fell down stairs on tournament eve.
Phil Mickelson had a 65 on Sunday and was at six under in a tie for 12th in a group which included Australian Stuart Appleby, who closed with a 69.
Johnson will be among the favourites to win the U.S. Open starting on Thursday at the Shinnecock Hills Golf Course in NY and he is confident of repeating his 2016 victory in the year's second major. "I like this golf course, and I feel like I play it well and wanted to be on top of my game going into next week, too".
Snedeker birdied four of his first seven holes and then three of his final four, including a 14-inch tap-in at the 18th. "I've been swinging really well and have a lot of confidence in my game, and I played really well all week".
That was claimed in style as Johnson spectacularly holed out from 171 yards on the par-4 last for an eagle and a six-shot victory. "I felt I did that pretty much all day today and hit a lot of great shots in there and had a lot of really good looks at birdies".
He previewed his dramatic finish on the par-5 16th.
He had won his only two previous starts at the event, the only PGA Tour wins of his career. Scott Piercy withdrew after nine holes at 5 over.
He had made three birdies and just one bogey as he stepped on to the final hole and in characteristically nonchalant fashion he proceeded to hole his second from 170 yards. And from that moment on, Johnson's second title of 2018 was never in doubt.