Johnson first heard about Marie Grover's 100th birthday through Twitter, where Jamie Klingler, of Philadelphia, tweeted about her attempt to get "The Rock" to wish her best friend's grandma a happy birthday on Sept. 22.
Klingler said Grover "adores" Johnson and that a friend of a friend was working on contacting him.
"She will lose her mind," Klingler said. "When we were in high school (25 years ago) she would wear a rock sweat shirt and tell us how handsome he is."
But Johnson saw the tweet and said it was his first time hearing about it, and asked her what she needed. She said a video of him singing "Happy Birthday" would surprise Grover and "be so fun." Grover said on Twitter that she had known the family since she was 11 years old and that she had called Grover "grandma" ever since.
Grover could not contain her laughter when she was surprised with Johnson's video message on her birthday, Oct. 1.
In the video, Johnson sang "Happy Birthday" to Grover and said that he was "honored" to have her as a fan.
"I'm sending you so much love and a huge congratulations on 100 years. What an amazing life," Johnson said in the message. "I, and I'm sure everyone around you right now, [is] very happy you were born. I'm so happy and honored to send you this message."
The next day, Klingler tweeted the video thanking Johnson for his help. Johnson later retweeted it, saying that "stuff like this is always the best part of fame."