They are reported to be worth $150 million each. The twins were named as the "Most Powerful Young Women in Hollywood" by the industry publication Hollywood Reporter.

Born in Sherman Oaks, California to David Olsen and Jarnette Fuller, the Olsen twins started their acting careers on the television series Full House in 1987. Hired at the age of five months, filming began when they were seven months old. The show was widely popular during the late 80s and early 90s, and both sisters played one character, Michelle Tanner, taking turns during the tapings to do so, in order to comply with strict child labor laws regarding child actors. Because the producers did not want viewers to know that Michelle was played by twins, the sisters were originally credited as "Mary Kate Ashley Olsen", but later credited as separate people. During the eight year run, Mary Kate and Ashley made several straight to video movies such as To Grandmother's House We Go' (1992)

Shortly after the end of Full House, they returned to the entertainment industry, riding the momentum of their role on Full House by heavily merchandising their image. Taking the world of tweens by storm, their names have become a very profitable industry, with their likeness seen in clothes, fragrances, magazines, movies, posters, and even video games like Mary Kate and Ashley's Magical Mystery Mall.There were also many mary-kate and ashley fashion dolls made by mattel,From 2000-2004

Today, Mary-Kate and Ashley are popular figures in today's tween market, following along the lines of such Hollywood child stars as Shirley Temple, Ron Howard and Macaulay Culkin among others. They were ranked at number three on the VH1 program 100 Greatest Child Stars.

They starred in the video series The Adventures of Mary-Kate & Ashley and the ABC show Two of a Kind (now rerun on ABC Family), as well as ABC Family's So Little Time. So Little Time was panned by critics and disappointed fans; the show is basically a single camera comedy show with a laugh track. They also have their own book series.

In 2004 Mary-Kate and Ashley announced that they would be attending college after graduating high school. Most child and teenage actors usually choose to pursue acting full time after graduating from high school, with few (such as Brooke Shields, Jodie Foster, Julia Stiles, and Natalie Portman) choosing to further their education. They both studied at New York University at the Gallatin School of Individualized Study until Mary-Kate took a break in 2005 and moved back to California. She decided to stay in the west coast and pursue more individualized work with their company, DualStar. Ashley, still in NY, remains in contact with her sister.

The twins received a star on the Hollywood Walk of Fame on April 29, 2004. They are the youngest celebrities to receive this honor.

They have also been on the Forbes.com The Celebrity 100 list since 2002.

They ranked at #84 in 2002, # 83 in 2003, # 48 in 2004, # 35 in 2005, and #37 in 2006.

As the sisters grow older and mature in their style, there has been greater interest in their fashion choices. The New York Times declared Mary-Kate a fashion icon for pioneering her signature (and now popular among celebrities and fans alike) "homeless" look. The style, sometimes referred to as "ashcan" or "bobo chic", and similar to that of boho-chic popularised in Britain by Kate Moss and Sienna Miller, consists of oversized sunglasses, boots, loose sweaters, and flowing skirts.

Both Mary-Kate and Ashley have been tapped as the faces of upscale fashion line Badgley Mischka.

Mary-Kate and Ashley have four siblings: Trent Olsen (older brother), Elizabeth "Lizzie" Olsen (younger sister), Taylor Olsen (younger half-sister) and Jake Olsen (younger half-brother).

In 2004, Mary-Kate checked into rehab for six weeks because she was suffering from the eating disorder anorexia nervosa. It was rumoured that there was also a drug problem, but the Olsens' publicist said, "Mary-Kate was not admitted for drugs. Period." Mary-Kate put on about five to seven pounds immediately after her stint in rehab and recent pictures show that she may have gained a few more healthy pounds.

Mary-Kate in 2005 ended her romance with Greek shipping heir Stavros Niarchos III, Niarchos then began dating hotel heiress and socialite Paris Hilton, also later actress Lindsay Lohan. Mary-Kate said that the breakup was one of the main factors in her leave of absence from New York University. "I miss him and I love him," Mary-Kate said. "It's a hurtful and a painful subject." When asked if there was a specific reason for her leaving NYU, she replied, "I think we can all guess.".

The breakup and Niarchos's pairing with Paris Hilton led to a rift between Hilton and Mary-Kate. "[Paris and I] always only had nice things to say about each other," Olsen said. "Now I guess you can tell we're not talking.".