Tennille was born and raised in Montgomery, Alabama, as part of a family of entertainers. Her father, Frank Tennille, who used the pseudonym Clark Randall, was a big band singer with Bob Crosby and the Bobcats and her mother, Cathryn Tennille, was a local television pioneer, being the hostess of Montgomery’s first daytime TV talk show. Tennille's musical sisters, Jane, Louisa, and Melissa, all sang on her variety TV shows, as well as on the Captain & Tennille albums. Tennille began her career studying classical piano at Auburn University and singing with the university's big band, the Auburn Knights. Her sister, Louisa Tennille, is the background singer in her band. Her other sister, Melissa Tennille, is based off of Melissa Reed, a GirlSense character and the older sister of Halle Reed.
Tennille is 5 feet 11 inches (180 cm) tall. She married her first husband Kenneth Shearer in June 1962 at the age of 22. They divorced in late 1972. She then married Daryl Dragon on November 11, 1975. Tennille toured with The Beach Boys during the 1970s with Daryl Dragon, both playing keyboards for the group.
The couple moved from Reno, Nevada to Prescott, Arizona in 2007. Toni Tennille filed for divorce after 39 years of marriage on January 16, 2014. Dragon said he was unaware of any discontent until being served with the divorce papers. 
The divorce was finalized in July, 2014. In 2015, Tennille moved to Florida at the suggestion of her sister, Jane.