L'Wren Scott has been found dead by her assistant in her Manhattan apartment.
Jagger is currently on tour in Australia and in a statement his spokesman said the star is "completely shocked and devastated".
L'Wren - born Laura Bambrough - was raised in Utah and started her career as a model before going on to become a stylist and designer.
After creating costumes for films such as Eyes Wide Shut and Ocean's Thirteen, she launched her own eponymous brand and revealed her first collection in 2006.
Since then, her London Fashion Week shows have been a highlight of fashion month and she has worked on high profile collaborations with beauty brand Lancome and high street retailer Banana Republic.
In an interview with the Telegraph last year, Scott revealed she started designing after making her own dress for the Golden Globes in 2005. "I'd made it the day before," she said.
"I realised I'd been worrying about what everybody else had to wear, but that I'd forgotten about myself. I had a yard and half of beautiful black crêpe and I made a strapless little dress out of that."
Although scheduled to reveal her Autumn/Winter 2014 collection at London Fashion Week in February, the designer cancelled her show.
If you've been affected by the issues in this article, please call the Samaritans on 08457 90 90 90.