The pair dated after working together on the 2008 musical movie, but later split.
Seyfried, 32, is now married to Thomas Sadoski, with whom she has a child.
She and Cooper play lovers in the Mamma Mia sequel – something which Cooper said was actually not too awkward.
“She’s a friend. It was always going to be OK. You just have to be delicate with those situations,” he told Attitude.
“She’s married now and has a beautiful child. We knew we were going to be working together [again] on a set, going back to a part which was certainly going to remind us of 10 years earlier, but I know her family, I know her mum, who was there a lot.
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“It was a pleasant atmosphere. It was really interesting to see how different our lives are after 10 years.
“It is a long time and it was nice to spend time [with her].”
Seyfried had recently said that her husband had some natural jealously over her working with her ex.
“I think he has the same kind of issues that I would have if we were hanging out with his ex-girlfriend from long ago,” she told The Mirror.
“I think it’s always like, ‘I’m so in love with this person, so that person must be so in love with her too’.
“I assume everyone’s in love with Tommy because I’m in love with him. I assume that everybody sees him the way I do.”
She added: “I’d guess he assumed Dominic is in love with me the way he’s in love with me. And it’s just not the case.
“It’s sweet. I’d rather he be jealous than completely fine with it. He’s been great.
“He’s very supportive and he loves this movie, and [he and Dominic] got along absolutely fine when we were shooting.”
Mamma Mia: Here We Go Again is out now.