Photos from the night show the star wearing a dark suit and a Halloween skull mask over his face on his way to and from the party.
From video obtained by TMZ, Ben appeared to be drunk and had trouble walking when he left the party.
The star then almost fell over in the street, and used a parked SUV to steady himself, before he followed a female friend into a waiting car.
The next morning, the star addressed his behaviour, confirming that he'd stumbled in his sobriety in further video footage obtained by TMZ.
Speaking to a waiting reporter, Ben said on Sunday morning: "Well you know, it happens, it's a slip."
However it seems the actor won't be giving up so easily.
"I'm not gonna let it derail me," he added.
Just hours earlier, Ben had addressed his sobriety mission on his Instagram, while also making a joke about recent media reports surrounding his relationships and dating.
“HA, you got me. I’m dating," he wrote.
"But let’s be serious for a moment and talk about something that is actually important.
“I have been in recovery for over a year and part of that is helping out others. @themidnightmission is an incredible organization that helps those in need with housing, training, development and recovery.”
“I’m making a donation today because there are people battling addiction every day that don’t have the resources and need help,” he continued.
"Who else is with me? Click the link in my bio to donate."
Ben, who split from his wife Jennifer Garner in 2015, has been battling his alcohol addiction for a number of years, and has been in rehab a number of times.
In a candid interview in March, the star opened up on Today.
“Being an alcoholic, it’s part of my life, it’s something that I deal with,” said the actor.
“It doesn’t have to subsume my whole identity and be everything but it is something that you have to work at.”
“I had a problem and I really want to address it and I take some pride in that,” he added. “It’s about yourself, your life, your family… we encounter these kinds of hurdles and we have to deal with them.”