People also wanted to see Johansson in an action role, something she's done more frequently in movies like "The Island" (2005), "Iron Man 2" (2010), "The Avengers" (2012) and "Captain America: The Winter Soldier" (2014).
"The main reasons for coming to see the film were the story, and then Scarlett," Rocco said. "She's very diversified in what she does. Luc brought out the best of her in this movie."
"Lucy," rated R, opens internationally next week.
Meanwhile, Johnson's draw with men, women and children helped "Hercules" power into the No. 2 spot. In addition to its domestic haul, the film brought in $28.7 million from Russia, Australia, Britain, Malaysia and other countries in Asia.
Megan Colligan, Paramount's head of domestic marketing and distribution, said Johnson once again proved his appeal as a global action star who fights bad guys with a "a wink and a smile."
"He can be very serious and menacing and imposing, and he has this ability to melt it all away, which is why women show up for a movie like this," Colligan said. "He has a lot of appeal for kids."
Johnson himself drove over half of the buzz on social media platforms like Twitter, said Benjamin Carlson, president of tracking firm Fizziology.
"They weren't talking about any of the co-stars or Brett Ratner, the director," Carlson said. "It was all about 'The Rock' with this film."
Despite the two films' solid performances, the summer box office is still down by around one-fifth compared to last year, according to Paul Dergarabedian, senior media analyst with Rentrak.
That's partly because one would-be huge performer, "Fast & Furious 7" was postponed from its July 11 release due to the death of actor Paul Walker, he said. This summer also lacked some other blockbuster sequels like last year's "Despicable Me 2."
That puts a lot of pressure on "Guardians of the Galaxy," a Marvel superhero film being released in the U.S. next week, to outperform, he said.
Even though it features lesser-known characters, including Chris Pratt as Peter Quill and Zoe Saldana as Gamora, it could still break out, Dergarabedian said.
"Any time you have a Marvel movie in the summer, it's still a factor," he said.
Another film that opened in wide release over the weekend was the Rob Reiner-directed romantic comedy "And So It Goes," starring Michael Douglas and Diane Keaton, which collected an estimated $4.6 million, according to distributor Clarius Entertainment.