Her acting career began at the age of nine months, sharing the role of Michelle Tanner with her twin sister Ashley Olsen in the television sitcom Full House (1987–1995).

In 1993, the production company Dualstar Entertainment Group was founded, which produced a long string of television films and direct-to-video releases featuring the girls; they starred in Passport to Paris (1999), Our Lips Are Sealed (2000), Winning London (2001), Holiday in the Sun (2001) and in the television series, So Little Time (2001–2002), for which she was nominated for a Daytime Emmy Award for Outstanding Performer in Children's Programming.

In March 2012, both Mary-Kate and Ashley officially indicated their interest to retire as actresses in order to focus on their careers in the fashion industry.

[5] She has an older brother Trent, a younger sister (actress Elizabeth Olsen), and two half-siblings Courtney and Jake (from her father's second marriage).

[9] After graduating from Campbell Hall while she’s 18 in 2005, Olsen and her twin sister went on to attend the Gallatin School of Individualized Study at New York University.

[10] Olsen began her acting career at the age of nine months old when she and Ashley were hired to share the role of Michelle Tanner on the popular television sitcom Full House (1987–1995).

They starred in Passport to Paris (1999), Switching Goals (1999), Our Lips Are Sealed (2000), Winning London (2001), Holiday in the Sun (2001) and in the television series, So Little Time (2001–2002).

They became household names and popular figures in the preteen market during the late 1990s and early 2000s, with Mary-Kate's likeness seen in clothes, books, fragrances, magazines, movies, and posters, among others.

These ventures, combined with an array of licensing deals for their names and likenesses, made Olsen wealthy at a young age.

Her one short scene was ultimately cut from theatrical release, but was included on the film's Unrated version on DVD and Blu-ray.

In 2011, Olsen appeared in the motion picture adaption of the Alex Flinn novel Beastly, which was her final acting project.

[20] In 2007, Forbes ranked the twins jointly as the eleventh-richest women in entertainment, with an estimated combined net worth of $100 million.

[22][better source needed] In 2004, they made news by signing a pledge to allow all the workers that sew their line of clothing in Bangladesh full maternity leave.

They head a designer fashion label, The Row, as well as "Elizabeth and James",[22] "Olsenboye", and "StyleMint"[24] retail collections.

[25] She tested the design on a variety of women of all body shapes and ages in an attempt to find a "commonality in fit and attitude.

[44] On July 12, 2019, Olsen and Fatum won a 1.25m two-phase showjumping class at the Longines Global Champions Tour in Chantilly, France.

[50] In May 2012, Olsen began a relationship with Olivier Sarközy de Nagy-Bocsa, half-brother of former French president Nicolas Sarkozy.

[63][64] After discovering Ledger dead in his bed on January 22, 2008, his massage therapist called Olsen twice before contacting police.

[65] Responding to a claim by an anonymous law enforcement official that she would not speak to federal investigators without a promise of legal immunity, Olsen's attorney Michael C. Miller said, "We have provided the government with relevant information including facts in the chronology of events surrounding Mr.