The 2023 champion opened up a four-stroke lead before the weather-marred first round was suspended under darkness on New York's Long Island.
As milestone-man Adam Scott led a disappointing Australian challenge, Clark cashed in on surprisingly easier afternoon conditions to surge to six under through 16 holes.
A two-hour fog delay kept the field waiting on Thursday before Scott ground out a three-over-par 73, in his 100th consecutive major championship.
The former world No.1 found himself nine shots off the pace after Clark burst from the pack with an eagle and two birdies in a brilliant back-nine stretch.
Clark will return early on Friday (Saturday AEST) looking to retain the first-round lead and potentially break the four-under first-round record at the famed Southampton layout.
"Eat as fast as possible, sleep as hard as possible, and get out here and hopefully keep playing good," he said.
Fellow Americans Sam Stevens, Max McGreevy and talented amateur Ryder Cowan all posted two-under 68s in the morning, before gusting winds subsided somewhat to allow Clark's blitz.
Only three players out of 654 have ever finished an Open at Shinnecock under par, and officials are expected to toughen up the layout over the remaining three days.
Already Clark will take some catching.
Another former winner, ex-world No.1 Dustin Johnson, was level pegging with Clark at four under before making a horror three-putt double-bogey right before the horn sounded to mark the end of the day's play.
Two more former US Open champions, Jon Rahm and Matt Fitzpatrick, were also at two under late in their rounds as golf's cream rose to the top.
Chasing the final leg of golf's career grand slam, world No.1 Scottie Scheffler was captured having an animated discussion with his long-time swing coach after starting with a two-over 72.
"Any time you get conditions like this, I think the scores are going to be high," Scheffler said.
"It's an interesting golf course. Part of the challenge so far is judging the conditions as well, judging how this course is going to play."
World No.2 Rory McIlroy opened with a 69, while JJ Spaun's 77 was the highest first round from a defending champion in 14 years.
Earlier, play was suspended for two hours after just 30 minutes of action as the fog encircling the course worsened to such an extent that players on the 12th tee in particular could not even see the landing area on the fairway.
Scott wobbled with a double-bogey six at the 16th hole but struck straight back with a birdie on 17 to post the lowest round among the five Australians.
Min Woo Lee was three over through 14 holes when play was stopped.
Lucas Herbert carded a four-over 74 and his LIV Golf Ripper GC captain Cameron Smith a 75.
Smith's round was blighted by a dreaded triple-bogey six at the par-3 seventh hole, when the former British Open champion needed two shots from a bunker and then three-putted.
Former world No.1 Jason Day's 14th Open tilt lasted only nine holes before he withdrew with a back injury.
A grimacing Day bogeyed seven holes to turn in 42 before the 2011 runner-up called for officials and was carted back to the clubhouse for treatment.
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