Moore died Wednesday from cardiopulmonary arrest after she had contracted pneumonia, her rep confirmed to ET.
Reiner says the last time he saw Moore was at a formal event, where the TV icon looked well, but he could tell that her health was waning.
"She had this childhood diabetes and had lost her sight -- practically all of it," Reiner recalls. "I came up to her, and she was looking at me, and she wasn't recognizing me until I spoke, and then she lit up. I realized she couldn't see me, and that was very sad."
According to Reiner, those close to the legendary actress were aware that she didn't have much time left.
"She spent her last months at home with hospice people around, not communicative," Reiner says. "We had known about it for a while, but you're never not shocked. When I turned on the television and learned Mary had gone, it was a shock. At 80. I'm 94 now, and I hope to go a few more years. I've got work to do."
Reiner and Moore's friendship dates back to when she was a fledgling actress in her early 20s, but even then, Reiner could tell that she had something special.
"She was one of these people -- and I think it's the fact that she was a dancer to begin with -- she was totally put together perfectly," he says. "If someone comes from Mars and says 'Show me what a human is,' or 'What does a human do?' we would show them Mary and Dick doing the soft shoe. It was my favorite moments from the show. It was magic."
Moore's big break came when she was cast as Laura Petrie on The Dick Van Dyke Show, where she was a trailblazer for women in television, but the role almost passed her by.
"She had failed twice that week," Reiner recalls. "She came in. She walked through the door and I saw those legs, that hair, those eyes. I gave her a page from the script and she read the first line. I heard what I called a ping in her voice, and I made my hand into a claw like they have at the arcades where you pick up candy. I grabbed her by the head and said, 'Come with me young lady.' I walked down the hall to Sheldon Leonard's office, our executive producer, and I said 'I found her,' and dropped her there. That was the beginning of a wonderful four or five years."
Since he wasn't able to be by her side when she passed, ET gave Reiner a chance to give a final message to Moore.
"Mary, you were the greatest," Reiner says. "You and Dick defined my life at one period, and I'll never forget you for what you did for me by being Laura Petrie."