Martha began her acting career in the Broadway play Annie at the age of eight where she played the role of July and understudied the title role. In May 1985, Martha got the role of Lily Walsh, daughter of the wealthy Lucinda Walsh on As the World Turns. She continued in the role until October 1989 when she took a break to pursue other opportunities. Martha returned to the role in April 1993.
Martha's hard work and dediction have earned her two Daytime Emmys, one in 1987 for Outstanding Younger Actress in a Drama series and in 2001 she won an Emmy for Outstanding Lead Actress for her portrayal of twin sisters Lily Snyder and Rose D'Angelo. She has been nominated four other times for an Emmy in the category of Oustanding Younger Actress in a Drama Series and in 2002 she was nominated again for Outstanding Lead Actress. She was also nominated for a Soap Opera Digest Award with co-star Jon Hensley for best couple.
Martha resides in New York state with her husband, New York Police officer Michael McMahon, and their children.