While appearing virtually on Jimmy Kimmel Live Thursday, Margulies, 53, opened up to host Jimmy Kimmel about how she celebrated her famous friend's milestone 60th birthday with a sweet and reflective message.
When asked by Kimmel, 53, if she had contacted her "old pal," Margulies detailed that she had indeed. "I wrote to him yesterday because I wanted to write to him on the last day of his 50s," she said.
Noting that she calls Clooney "Doug" and he calls her "Carol" — the names of their respective ER characters — Margulies continued, "It's just something we've always done."
"And I said, 'On this, your last day of being 50, I just want you to know that when you're 90 and you look back and see how young 60 was, you will curse the day you wasted time thinking you're getting older or that you're too old.'"
Clooney celebrated his 60th birthday on Thursday, despite previous sentiments shared by the actor that he wasn't exactly looking forward to the upcoming occasion.
During an interview with Entertainment Tonight last month, Clooney shared that he doesn't view his birthday as the most exciting thing.
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"As far as turning 60, listen, I'm not thrilled with it but it's better than dead," he joked. "So, I'll take it. I got two options. The older you get, the lower your bar gets."
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On Thursday, Margulies also paid tribute to Clooney on social media, sharing a throwback photograph of the two on Instagram.
"Happy Birthday to my dear friend who reminded me yesterday that we have known each other for 27 years!" the actress wrote alongside the shot, which she noted was "taken for the cover of TV Guide on the back lot at WB's studios during a lunch break."
"George is a stand up fellow who walks the walk and reminds me all the time that doing the right thing is always the braver choice. ❤️❤️❤️," she continued.
While chatting with PEOPLE in April, Margulies opened up about meeting Clooney on the set of ER, a series in which she starred on for six seasons.
"He could not have been kinder or sweeter," the actress said of Clooney, who starred on the show's first five seasons. "... With George and me, it was so organic. I was just supposed to be a guest star, number 39 on the call sheet. But he treated everyone the same."