Casey, the World Number Nine, shot an opening one under par 69 at Pebble Beach Golf Links to move into a three-way overnight lead with 2003 US PGA Championship winner Shaun Micheel and Zimbabwe's Brendon De Jonge, whose round included a 91 yard hole-out for eagle at the par five 14th.
The leaders hold a one stroke lead over a six-man group that contains another Englishman, Ian Poulter and Spain's Rafael Cabrera-Bello.
Casey had been two under after five holes but dropped a shot at the ninth before finishing birdie, bogey, par, birdie and the 32 year old was delighted with the finish to a solid day's work spent battling some indifferent ball-striking.
"Even today I struggled with it, missing it both ways, which isn't great," Casey said. "But I did a wonderful job of managing my way around this golf course.
"I had low expectations coming in because of the ball-striking and practice had been certainly frustrating and I went out there thinking just have a great time, enjoy the crowds, enjoy Pebble Beach, it's a wonderful place to be any time of the year, let alone when it's gorgeous sunshine.
"And it paid dividends. The putting was obviously very good today. That was the key to the round. I made the putts and made the par saves when I needed to."
Poulter was also satisfied having limited his round to a couple of over par scores on a difficult day on Pebble Beach's firm fairways and small greens that produced a combined two eagles and 341 birdies against 730 bogeys and 132 double bogeys or worse.
"I just played really solid," World Number Eight Poulter said. "I didn't miss many greens and drove it well.
"I gave myself plenty of looks from the right side of the pins. You need to do that around this golf course. It's all about distance control with your irons. You can drive it well and have a few looks at birdie and you can put together a good score.
"One under is a good score today.
"If you're going to limit your bogeys to two a day around this golf course, you're going to have a lot of shortish iron shots...and I think that's probably five or six chances out there."
Poulter shares second place with Korea's KJ Choi, Canada's Mike Weir, Spain's Cabrera-Bello, Alex Cejka of Germany and 18 year old Japanese star Ryo Ishikawa, while another Englishman, Luke Donald, and Northern Ireland's Graeme McDowell are in a group a further shot back at level par.
Cabrera-Bello’s efforts were all the more astonishing given it was his first Major and he had his Visa waiver refused at Madrid airport.
He eventually got a later flight, only for his clubs to go missing.
“I had qualified and was desperate to get started and I didn’t have any clubs,” said the Austrian Open winner.
“I had to walk the course on Monday with a few wedges I borrowed and my clubs arrived on Tuesday.
“Today this course, if you lost your concentration for a minute it’s going to beat you and hit you very hard. I really tried to stay calm and focused on my task and I think I did that.”
McDowell added: “I think level par will be score to winning is this wind keeps up. It’s a tricky dynamic with a cool sea breeze and linksy feel. I would certainly take level par right now.”
World Number One Tiger Woods did not drop a shot until the par four ninth but two more bogeys, at the 16th and 18th, left the man who tamed Pebble Beach at the 2000 US Open Championship with a birdie-free 74, three over par.
That was the same score as playing partner and World Number Three Lee Westwood, with two-time champion Ernie Els the best of the marquee group at two over following a double bogey five at the 17th.