Theobald initially gained fame at age 21 after she was attacked by a lion during a BMW photo shoot at the International Automobile Show at the New York Coliseum on April 9, 1966. At first the lion named Ludwig was calm in posing with the model, but when he was later pulled out of its cage, apparently aggravated by his trainer, he bit Theobald in the left thigh. She was taken to Roosevelt Hospital and underwent surgery to save her leg from being amputated. A spokesman for the International Automobile Show stated no live animals were supposed to take part, but the show had no control over separate promotion for the event by automobile companies previewing their cars. Theobald initially sought $3 million in a lawsuit against the International Automobile Show, Grey Public Relations, Dawn Animal Agency, Coliseum Exhibition Corporation and Hoffman Motor Corporation, but received $250,000 in a settlement in 1971.
She later drew notoriety for obsessively stalking opera singer Birgit Nilsson for nine years, beginning at the summer of 1968 at the Bayreuth Festival and ending upon Theobald's suicide in 1977. Theobald sent her daily flowers and followed her around the globe. Nilsson did not know Theobald's real identity, only knowing her as Miss N, until 1971, when she read an article about the lawsuit settlement and saw her photo. Following her settlement, Theobald's obsessive behavior with Nilsson only intensified, as she had more money to travel and send gifts. Nilsson recounted her experiences with Theobald at length in her memoir La Nilsson which was published in 2005.
As an actress, she appeared on the New York stage, in television commercials, and made some minor appearances in films and television shows. She committed suicide on August 20, 1977, in New York City.
- Ralph Blumenthal (May 21, 2006). "Soprano's Tale: Obsession, Love and Death. Offstage". The New York Times. Retrieved August 6, 2009.
- Social Security Death Index, 1935-2014. Social Security Administration.
- "Girl Bitten By Lion Shows Improvement". The New York Times. April 10, 1966. Retrieved 5 August 2017.
- "Model Mauled by Lion Asks $3-Million in Suit". The New York Times. June 14, 1966. Retrieved 5 August 2017.
- "Model Bitten by Lion Here Takes $250,000 Settlement". The New York Times. 8 April 1971. Retrieved 5 August 2017.
- Blumenthal, Ralph (21 May 2006). "Soprano's Tale: Obsession, Love and Death. Offstage". The New York Times. Retrieved 5 August 2017.
- Kathryn Leigh Scott (August 2009). "The Star and the Stalker". Opera News. Retrieved August 6, 2009.