Swift, who has a small role in the film, took to social media after hitting the red carpet to commemorate the big night.
"It was so inspiring getting to work with this awesome team and just surreal to be at this premiere," he captioned a post on Instagram, along with the film's poster.
The crime drama, which stars Elle Fanning, Zoe Saldana, Brendan Gleeson and Ben Affleck, who also wrote and directed the film, hits theaters on Friday.
ET caught up with Affleck on Monday, where he opened up about the joy and pride he felt in watching his younger brother, Casey Affleck, win his first Golden Globe on Sunday night.
"The overwhelming sense I had last night, watching my brother win, and believing in him for so long and knowing how talented he is… it was the most memorable experience in my life," the 44-year-old star shared. "I'm thrilled. I wish him the best, but already what he's done is amazing."
For more on the famous brother duo, watch the clip below.