Originally Monika Jagaciak, "Jac" was born in Poznàn, the chief city of Wielkopolska (Polish for "Greater Poland"), Poland in 1994. She comes from an incredibly athletic family. Her sister, Anna, who was named one of Poland's greatest long jumpers, was a medal winner in the European Athletics Junior Championships in the long and triple jumps. Their father coaches sprinters, and mother is a physical education teacher.
Jac began modeling at the very young of 13 in 2007 when she attended open casting for GAGA Models Agency in Poland. That summer, she signed with IMG, which is the same agency she is with today. She immediately appeared on the cover of the French Magazine, Jalouse. Then, by the end of the year, Jac landed a gig in the Hermes fall/winter fashion collection. However, she was quickly met by a bit of controversy early on involving with Australian Fashion Week because of how young she was, sexualizing women in fashion at an early age, of which she was of 14 at the time. She had to sit out of a number of shows due to her age.
By 2008, Jac was making her name known internationally, traveling to Japan and nabbing the cover of Elle. Shortly after getting picked up for a cover of Harper's Bazaar, she was met again with the age issue when her scheduled cover with Australian Vogue was pulled due to age restrictions. Jac took a short two-year break and returned strong with New York Fashion Week, picking up modeling gigs with the likes of Calvin Klein and Donna Karan, making Jac and her impressive runway strut one of the hottest fashion industry commodities of 2009. She began walking for big brands like Marc Jacobs, Versace and Prada. She was named one of the "Top Ten Models to Watch" by New York Magazine, meanwhile getting her face on the covers of more than a dozen big time magazines.
In 2011, Jac appeared in several Vogue editorials, as well as signed with Yves Saint Laurent Saharienne fragrance. Then in 2015, she was picked up as one of Victoria's Secret's newest Angels. Jac lives with her family in Mosina, just south of where she grew up but has also been living in New York since she was picked up by Victoria's Secret in early 2015.