The Suicide Squad star (26) had some alterations made to the vintage piece originally worn by her mother Sarie Kessler.
Australian designer Casey Tanswell was recruited for the alterations but didn't know her client was a superstar until she arrived for the appointment.
"She didn’t know it was Margot until she turned up at her home for the fitting," a source told the Courier Mail.
Margot, who splits her time between her home in London and Los Angeles, has been dating assistant director Tom since meeting in 2013 on the set of World War II drama Suite Francaise.
She recently gushed to Vogue about falling in love with her now-husband and described him as the "best looking guy in London".
"I was the ultimate single gal. The idea of relationships made me want to vomit. And then this crept up on me," she explained.
"We were friends for so long. I was always in love with him, but I thought, 'Oh, he would never love me back. Don’t make it weird, Margot. Don’t be stupid and tell him that you like him.'
"And then it happened, and I was like, 'Of course we’re together. This makes so much sense, the way nothing has ever made sense before.'"
Margot kept details of the wedding under wraps but confirmed the reports days after with this cheeky Instagram snap.