Ahern was born on June 25, 1920, in Los Angeles, California, and she was the daughter of Fred and Elizabeth Ahern. She was the third in a family of four children, one of her siblings was Peggy Ahern.

Ahern got her acting career started in 1923 in the silent film The Call of the Wild, produced by Hal Roach. Also making her first screen appearance was her older sister Peggy Ahern. It was the actor Will Rogers who recommended that Ahern's father put Ahern and her sister in show Business. She appeared in numerous Our Gang films under the Hal Roach Studio, including Cradle Robbers, and was one of the last surviving Our Gang members.

In 1927, Universal Studios was in the process of making Uncle Tom's Cabin. Not satisfied with the boys who auditioned for the role as Little Harry, Ahern was contacted and requested to take the part. Her acting in the film turned out to be a success, and she won the best reviews of her career. Despite this, her career as a child Actress ended the same year, with Little Mickey Grogan being her last silent film and her only movie to feature her in a starring role (alongside Frankie Darro). A crowdfunding campaign was started in 2016 to Finance a restoration project for the film, following a similar campaign in 2015 to acquire a digital copy of Little Mickey Grogan in Paris.

In 1932, she teamed with her sister Peggy and started putting up performances that included dancing, singing, and playing instruments. The duo was billed as "The Ahern Sisters" and mostly appeared in nightclubs and hotels. Lassie later went on to work as a dance Teacher at the Ashram Health Spa, where many known stars were students. During the 1970s, she made several guest appearances in television shows such as The Odd Couple.

Ahern died in Prescott, Arizona on February 15, 2018 of complications from influenza at the age of 97. At the time of her death, she was one of three surviving silent era Our Gang members.