The Greatest actress wore the gold band - positioned next to her engagement ring on her left hand - for the first time in public. She married Mumford and Sons frontman Marcus Mumford last month.
Mulligan, who was appearing at the Met Ball in New York City, was wearing a sparkly mermaid-like sequin dress.
The Hollywood star told reporters of her outfit: "I'm really honoured to be wearing it."
Jake Gyllenhaal, Sienna Miller and Colin Firth were among the star guests present at their ceremony in Somerset in late April, while Adele performed at the couple's wedding reception.
They announced their engagement in July last year after five months as an item.