While the marriage will be her first, Mary-Kate's fiancé was married to fashion writer and author Charlotte Bernard for 14 years. Olivier and Charlotte, who have two children Julien, 12 and ten-year-old Margo, divorced in 2011.
Mary-Kate and Olivier, who are regularly seen together at basketball games in The Big Apple, have received plenty of media attention since going public with their romance in May 2012.
Fashionista Mary-Kate first sparked engagement rumours in January 2013 when she was spotted with a ring on her left hand during a trip to Paris, although shortly after the holiday a source told Us Weekly that the Olsen twin wanted to take her time with the relationship and plans to marry.
Later in the year, however, Mary-Kate was seen shopping for engagement rings on more than one occasion.
The actress-turned-designer is said to have first visited the Neil Lane jewellery shop – whose creations are a favourite with Hollywood's biggest stars – in November 2013.
Mary-Kate then returned to the high-end Los Angeles jewellers several times, although she was unaccompanied by Olivier.
The couple have been spending lots of time together in LA while Bennett makes the final cuts to his new film and were spotted on a dinner date at Dan Tana's in West Hollywood on 24 January.
Ashley and Bennett also supported each other at the funeral of Philip Seymour Hoffman in New York. Bennett was a close friend of the late actor having directed biopic Capote, the film which earned Philip an Oscar in 2005.