Born in Burlington, Ontario, Lally Cadeau made her acting debut in 1980, with a guest appearance on King of Kensington and a role in the TV movie Passengers. She also showed her comedy chops in the sketch-comedy series Bizarre, which lasted for five years. A year later, she appeared in the movie Threshold and played the role of Kate Brown in the CBC sit-com Hangin' In. That same year, ACTRA gave her an award for "Best Newcomer". She went on to have a role in David Cronenberg's Videodrome, appeared in the TV movies Between Friends and Kane and Abel. She also guest-starred on Street Legal and The Twilight Zone and did the movie Separate Vacations.

The 1990s found Lally in the role of Janet King on Road to Avonlea. "I sometimes realize that I am playing my grandmother in many ways, that Celtic soul that was so apparent in her," she said in a 1995 interview for the Avonlea Traditions Chronicle, in regards to how she played Janet. "I have Celt in me...and I pour that into Janet...". In 1995, she won the Gemini for Best Actress in a Continuing Leading Dramatic Role. Also in the 1990s, she appeared in two TV movies (Deadly Matrimony and Rossini's Ghost) and had a recurring role in the animated series X-Men. In 1998 she reprised the role of Janet in Happy Christmas, Miss King and continued to do voiceover work. She had a guest voice spot in the animated series Silver Surfer and has lent her vocal talents to episodes of Life and Times and to various commercials.

In more recent times, Lally has been busy doing theatre. For the last several years, she has been a fixture at the Stratford Festival in Ontario, performing in a canon of plays from Shakespeare (Romeo and Juliet and King John, to name a couple) to Tennessee Williams (Cat on a Hot Tin Roof, which she also did for Theatre Neptune in Nova Scotia) to one-woman shows (The Human Voice and Ruth Draper on Tour).

She lives near Stratford with her video-producer husband and three children.