The Shame actress, who married Mumford and Sons frontman Marcus Mumford last month, put the custom Prada dress up for sale on eBay.
© Rex Features / Matt Baron/BEI
The gown itself is made of silver and gold metal paillettes. While attending the ball, Mulligan told reporters: "I'm really honoured to be wearing it."
The bidding started off at $500, but has since risen to $2,025. The auction will continue until May 19.
The proceeds from the sale will go to Oxfam America.
Mulligan is reportedly in the running to play Nora Charles in Johnny Depp's Thin Man remake. The original films starred William Powell and Myrna Loy as the detective couple.
