Born in Santa Monica, California, the youngest of four children, with two older brothers Larry and Christopher (fourteen years older) and a sister Diane. Her brother Christopher was also a child actor, perhaps best known for his role in The Man Who Knew Too Much.

After landing a number of supporting roles, most notably, Ironside, Gunsmoke, and Julia, at age seven she was cast as Cindy on The Brady Bunch. She was the youngest actress ever to appear in the series.

As an adult Olsen has expressed that portraying Cindy made peer relations difficult for her as a child. Olsen has said that the episode she dislikes the most is the "tattletale" episode in which Cindy snitches on her brothers and sisters. Because of the episode, she was shunned by her real-life peers who did not understand the difference between actors and their characters.

Olsen has appeared in all "Brady Bunch" reunion movies, with the exception of A Very Brady Christmas in 1988, because she was on her honeymoon with her first husband. In that movie, Cindy Brady was played by Jennifer Runyon.

On Sunday, April 22, 2007, Olsen and her fellow cast members were honored with the TV Pop Culture Awards on the TV Land Awards. It was noted that this is the first award that the Brady Bunch has ever won.

During The Brady Bunch production, it was noted that even though her 'brady series father,' star Robert Reed, was suffering a great deal of unhappiness (both on and off the set), Olsen continued to develop a wonderful on- and off-screen relationship with him, along with Florence Henderson, who played her TV mother.

During a hiatus, she, along with Reed and other castmates, would often go out on vacations. One such vacation was a trip to Kings Island Amusement Park in Mason, Ohio, during the fifth and final season. At the end of that season, she was very aware of Reed's misbehaviour, off the set, mainly in relation to not speaking scripted dialogue (poor character dialogue being a point of an ongoing, long-term argument between Reed and series - Writer, Creator, Producer - Sherwood Schwartz). Due to an endplay argument with Schwartz, Reed was written out of the script and did not appear in the final episode of the series, "The Hair-Brained Scheme".

Olsen remained close to Reed until his death on May 12, 1992. She, along with her Brady Bunch castmates, attended Reed's funeral, with the exception of her best friend and Robert Reed's real-life daughter, Karen Reed (who guest-starred on one episode of The Brady Bunch) and her TV father's real-life mother, Helen Reitz.

As an adult, Olsen moved into the graphic design business and in 1998 briefly marketed a brand of shoes for Converse that glowed in the dark. She also worked as a talk show host at Los Angeles radio station KLSX from 1995 - 1996 with Ken Ober and co hosted and co-wrote another radio show with Comic, Allan Havey at Comedy World in 2000.

She appeared on Cartoon Network's talk show Space Ghost Coast to Coast in its twenty-sixth episode "Switcheroo" with Cassandra Peterson as "Elvira, Mistress of the Dark". Olsen has also been an advocate for migraine sufferers since 1998. She described her headaches on Larry King Live.