[1] Byrne began acting at a young age when she joined the cast of the Broadway musical Annie, where she played the role of July.

She was a featured actor on a number of prime time episodic television programs, including Kate and Allie; Murder, She Wrote; Parker Lewis Can't Lose; Jake and the Fatman; In the Heat of the Night; and Hearts are Wild.

Byrne is best known for portraying Lily Walsh Snyder on the CBS Daytime soap opera As the World Turns.

Byrne soon became part of a "supercouple" when then-head writer Douglas Marland paired "poor little rich girl" Lily with hard-luck farmhand Holden Snyder, played by Jon Hensley.

The show attracted controversy in 1987 when Byrne—months shy of her 18th birthday—and equally underage actor Brian Bloom played out a storyline in which they lost their virginity to each other.

The show's executive producer Christopher Goutman stated, "We made Martha an incredibly generous offer in hopes that she would remain a valuable member of the ATWT cast.

[citation needed] Byrne's last airdate on the show was April 22, 2008; her successor, Noelle Beck, appeared in the role of Lily from May 2008 until the series' end in 2010.

Byrne married Michael McMahon, a former undercover officer in the New York City Police Department, on November 12, 1994.

An episode of the TLC series A Baby Story chronicled her second pregnancy and the birth of her son, Maxwell Vincent McMahon, on September 3, 2002.