The couple - who were first rumoured to be dating in June - made their first public outing at the Macallan and Annie Leibovitz Photo Launch in New York City on Wednesday (October 10).
Kartheiser told Us Weekly of the romance: "I'm a really lucky guy!"
The actor went on to insist that he and Bledel always make time for each other in spite of their busy schedules.
"The most important thing, for us, is to make sure that we see each other in the flesh at least once every 10 days," Kartheiser noted.
"All the other stuff is good, like FaceTime and the texting... I love seeing her. You gotta make the flight!"
Bledel guest starred in Mad Men's fifth season as a mentally unstable housewife who has an affair with Kartheiser's character Pete Campbell.
Kartheiser previously dated The Hairy Bird co-star Rachael Leigh Cook in the 1990s, while Bledel was linked to Milo Ventimiglia from 2002 to 2006.
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