The actress attended the British Film Industry Reception at the royal's Windsor Castle home, but had no idea she'd actually get to meet Elizabeth Ii.
She explains, "I thought I would see the Queen from a mile away and take a photo on my phone or something, and then I arrived at Windsor Castle and someone said, 'Ms. Mulligan, would you follow us...' and I went into this room and there were just five people in there. And they said, 'You're all going to meet the Queen on your own'.
"I did my own hair... I just completely freaked out. And then this guy came up to me and he said, 'Ms, Mulligan, I just wanted to let you know the hem of your dress has fallen down. Would you like a safety pin?' And I was like, 'Yes, I need a safety pin!'
"If I met the Queen without a hem on my dress... my mother would behead me.
"He ran off and he got the safety pin and came back and he got down on the floor... and I was like, 'I need alcohol...'"
The booze failed to calm her nerves and she turned to actor/director Kenneth Branagh, who was also a castle guest, and asked him for a last-minute etiquette lesson.
She adds, "He's, like, practically related to them (royal family) and I said, 'Kenneth Branagh, what do I do?' and Kenneth Branagh taught me how to curtsy. It was a very brief lesson but he made me feel very comfortable."