The pair started dating in December 2012 and reportedly broke up in December 2017, according to E! News, citing an anonymous source.
It was only a few months ago in October when Dunham wrote lovingly of Antonoff on Variety, saying he taught her the importance of making political statements, among other things.
"He showed me the joy of collaboration. He showed me the magic of his home state of New Jersey and turned the butt of my snotty New Yorker jokes into a glimmering fantasyland," Dunham had written at the time.
Also in October, the couple celebrated their five and a half year anniversary together. It's safe to say that the breakup comes as quite of a shock, especially since the two have been embroiled in engagement rumors fairly recently. Dunham even admitted via a tweet in November 2017 that she thought Antonoff was going to propose after overhearing him and sister having a discussion.
In August 2017, Dunham and Antonoff shared adorable moments of them at the MTV Video Music Awards via social media. Antonoff was attending the Los Angeles show while on FaceTime with Dunham, who was presumably tuned in from her home in New York. Dunham posted screenshots from their FaceTime session including one where Antonoff was eating a banana, and one where he's eating a sandwich.
The last public event the couple went to was nearly a year ago during the Grammy Awards pre-party.
Considering how many sweet moments the couple have had together on social media, it's hard to think at the time that they would ever call it quits. But thankfully it wasn't an ugly breakup, according to the aforementioned source.
"It was mutual. Jack and Lena were growing apart and it just made sense for them to end their relationship where it was."
The source also said that despite the breakup, both were supportive of each other.
"They want the best for each other no matter what. They are both moving on."
A representative for both Dunham and Antonoff has reportedly confirmed the split to E! News.
Despite their split, it's safe to assume both Dunham and Antonoff will be busy with their respective projects. Dunham with her Lenny Letter feminist newsletter and Antonoff with his band and his producing gigs.