Johnson and Landry shared the lead at 4-under entering Saturday, but Landry had yet to play his second round due to a series of weather delays on Thursday.
Johnson opened 67-69 after playing 36 holes Friday for the one-shot lead.
Landry's 66 paced the field after round one and was the lowest ever opening-round score in a U.S. Open at Oakmont.
Scott Piercy, Sergio Garcia of Spain, Shane Lowry of Ireland and Greg Bourdy of France are tied for third at 2-under. Piercy, Garcia and Lowry all had second-rounds 70s while Bourdy surged into contention with a 67.
"It was a beautiful golf and I knew at the beginning of this week, I could do something well," said Bourdy, who opened with a 71. "I feel very good in my game the last few weeks, and I really love the difficulty of this course."
Summerhays scored a tournament-best 65 in his second round Friday, two off the course record set by Johnny Miller in winning the 1973 U.S. Open.
Louis Oosthuizen of South Africa also carded a second-round 65 Saturday to pull even for the tournament and is in a five-way tie for 11th.
Top-ranked Jason Day bounced back from a first-round 76 to card a 69 in round two with three birdies and two bogeys. He is 5-over 145 and in a large group tied for 45th.
"Obviously, the second round was nice," Day said. "The first round, obviously, was terrible."
Jordan Spieth kept his dreams of a U.S. Open repeat after matching 72s in the opening rounds.
"I think we're capable of shooting 4-under on the weekend, so that's our goal," the world's second-ranked golfer said. "That's what we're going to try and do these next two rounds.
Spieth won last year's Open at Chambers Bay in University Place, Wash., claiming a one-shot victory after Johnson three-putted on the 18th green.
Spieth is tied for 35th at 4-over 144.
Fourth-ranked Bubba Watson had four bogeys and a double bogey in shooting a 76 on Saturday, but he is safe for the weekend, also at 5-over.
McIlroy, the world's No. 3, exits at 8-over 148 after opening with four birdies on seven holes to come in at 4-under on his first nine, only to have a pair of double bogeys on the back at Nos. 3 and 9 for a 71.
Fifth-ranked Fowler missed his second straight cut in a major, leaving at 11-over 151 following Saturday's 75.
Mickelson finished at 7-over 147. The beloved 46-year-old lefty posted scores of 74 and 73, making 10 bogeys with only three birdies.
It's the first time since the 2007 U.S. Open and Open Championship that Mickelson has missed consecutive major cuts. He also missed the cut at this year's Masters.
"I don't feel like I have to search for anything on the range," Mickelson said. "I feel like my game is there, but my scoring hasn't been what I want the first two days."
Angel Cabrera of Argentina, who won the last U.S. Open at Oakmont in 2007, barely made it to the weekend, sneaking in at 6-over despite a second-round 76.
The top 60 players, including ties, made the cut for the weekend. The third round teed off at 3 p.m. ET Saturday.
NOTES: The United States Golf Association announced Oakmont Country Club will host the U.S. Open for a record 10th time in 2025. "(It) is testament to the quality of this fine golf course and the longevity of the strong relationship the USGA has with the club," USGA president Diana Murphy said in a statement. Also, the USGA selected Shinnecock Hills Golf Club in Southampton, N.Y. -- site of the 2018 Open -- to host the event for a sixth time in 2026. ... Henrik Stenson of Sweden withdrew from competition Saturday. "Minor neck and knee issues, nothing major. Hope to be back in action next week," he said on Twitter. Stenson, No. 7 in the world, finished at 9-over. ... Jon Rahm of Spain was the only amateur to make the cut. "It's a huge, huge accomplishment," the top-ranked amateur said. "Extremely honored to have done it."