Mary-Kate Olsen is an American actress, fashion designer, producer, author, equestrian, and businesswoman. She co-founded luxury fashion brands The Row, Elizabeth and James, and the more affordable lines Olsenboye and StyleMint alongside her twin sister Ashley Olsen.

She and her sister wrote a book called Disney's Once Upon a Time with Mary-Kate & Ashley.

She also wrote another book called Mary-Kate & Ashley's Walt Disney World Adventure (Mary-Kate and Ashley in Action). She also played Natalie in the "Help!" episode of the ABC TV show Samantha Who? and appeared in All My Children. Mary-Kate retired from acting in 2012 to focus on her fashion career with Ashley.

Elizabeth Olsen is her younger sister.