The former "Smash" star previously revealed her childhood connection with the new addition to the British royal family in January, when she shared a throwback snap of herself with the former "Suits" actress on Instagram.
"Meghan and I did musicals together as kids," Katharine explained alongside the image. "She grew up to be the Duchess of Sussex and I grew up to star on the West End, so same life if you ask me."
Katharine, who is currently starring in the "Waitress" musical in London's West End, was given the opportunity to reconnect with Meghan backstage at the WE Day charity event in the British capital last week (ends March 08), but the 34-year-old has yet to be given a copy of the snap.
"I don't have a picture even though there is one out there," she admitted on U.K. show "Good Morning Britain" on Tuesday, March 12, "because, you know, the royals have to approve everything, (but) I did get to reunite with her, which was so sweet!"
During the interview, Katharine opened up further about her history with Meghan, who is heavily pregnant with her first child, and reminisced about their school days together in Los Angeles.
"I went to school with Meghan Markle," she shared, referencing the Duchess by her maiden name. "I was in middle school when she was in high school. My sister was actually in her class, so they were the same age."
Recalling Meghan as a budding actress, Katharine added, "She was always lovely and always talented and always put together. I did a couple of shows with her, but I was younger and wasn't allowed to be in the more major roles with shows."
She then touched on the Instagram picture she had posted of the young pair earlier this year, showing the girls posing backstage, wearing make-up and costumes for a production.
Katharine admitted that she had considered posting the snap when Meghan wed Prince Harry last May (2018), but felt it was too "opportunistic," so she saved it and decided to release the old photo while she "had something to promote."
She has since extended an invite to the pregnant royal to attend her musical, which officially opened last week.