There have been rumors that they were having problems, with one report claiming Westfeldt wanted to marry and Hamm was not on the same page.
Jon and Jennifer released a joint statement announcing the split on Monday, but did not specify the reasons.
They said in a statement, “With great sadness, we have decided to separate, after 18 years of love and shared history. We will continue to be supportive of each other in every way possible moving forward.”
Hamm, 44, and Westfeldt, a 45-year-old actress and screenwriter, were last spotted together on July 22 at a party in New York.
There were rumors in April that Jennifer wanted to end the relationship just before Hamm entered rehab for his drinking issues for 30 days.
Hamm’s AMC series Mad Men ended in May after seven seasons on the air.
Jon previously stated that playing the role took its toll on him.
Just weeks before he secretly checked himself into rehab, the actor said, “Playing this guy does not come without its own difficulties, it’s not fun to live in this guy’s headspace year after year.”
He added, “People ask, ‘What’s the difference between you and Don?’ Look, I drink, I get drunk. I’m not immune to that.
“And (creator) Matt (Weiner) will tell you, the darkness in Don has not abated, it’s gotten worse year after year. It’s relentless. And it can be hard on you as a person.
I love coming to work; I love the people I work with. But it’s been rough.”
Hamm and Westfeldt started dating in 1997, but never married and do not have any children together.
In May, Jon appeared to backtrack on previous statements about marriage.
They worked together on the indie comedy Friends with Kids in 2011. Westfeldt wrote, directed, and starred in the film. Hamm was the co-producer and also had a small role in the movie.
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