Three years after Kaley surprised fans with the news that the pair had quietly dated, Johnny has finally opened up about the past romance.
"We're dear friends, still," he revealed in the December issue of CBS Watch! magazine. Kaley's not just an ex, she's part of my life."
So why has the 38-year-old not discussed the romance before?
"I just don't like to speak about it," he said. "And not just because I'm trying to be enigmatic; I just worry that it will conflict with people's acceptance of Leonard and Penny," he said of the pair's characters on the show.
"I get the curiosity, but I don't want it to distract from the story."
Johnny and Kaley, 27, have co-starred in The Big Bang Theory since it began in 2007.
In September 2010, Kaley revealed their relationship for the first time.
"We dated for almost two years," she told CBS Watch!. "It was such a huge part of my life and no one knew about it.
"This is the first time I've ever talked about it, ever. It was a wonderful relationship, but we never spoke a word about it and never went anywhere together.
"(We were) so protective of ourselves and the show, and we didn't want anything to ruin that.
"We knew we weren't destined to be together," she continued. "We accepted it and said, 'Look, if we ever break up, we will be professional.
"I'm so lucky it was a mutual ending and that we don't hate each other. We've seen that happen on other shows."
Since the end of their relationship, both Johnny and Kaley have stepped out publicly with new romances.
Johnny has been dating former Pan Am actress Kelli Garner since March 2012.
Kaley, meanwhile, announced her engagement to tennis player Ryan Sweeting after three months of dating in September.
This week, the happy couple got 'married' during Kaley's appearance on The Ellen DeGeneres Show.
The TV host gave the bride away during the fake televised wedding, and even offered up her own wedding ring.
During her interview beforehand, Kaley spoke about the moment she knew Ryan was 'the One'.
"By day two it was unbelievable, I said, 'This is it'," she revealed. "I know it looks a little crazy to some people!"