Mary graduated from RLHS in 1965. With no competitive girls sports at that time, she was active in GAA all four years and served as treasurer her senior year. She attended UW-LaCrosse and earned a BS in Physical Education. During her four years there, she again was ahead of the opportunity for women to compete athletically. However there was a club team in field hockey on which she played for three years being selected once to play on a Midwest All-Star Collegiate team that participated at the National Field Hockey Tournament in Atlantic City, NJ. O'Brien was also able to compete in various events called Sportsdays held at Northern Illinois University in volleyball and basketball while at LaCrosse. As part of her major requirements, Mary took an officiating class which eventually led to her being trained, tested and qualified at the National Level for Track and Field. Before graduating from LaCrosse, she was instrumental in starting the girls high school state track meet in Wisconsin. After graduating and moving to Illinois, she went on to serve as the starter for the first ever Illinois state track and field meet for girls, and served in that capacity for a total of 24 years. O’Brien began teaching at Larkin High School in Elgin, IL in 1969. When the Illinois High School Association sanctioned opportunities for girls to compete interscholastically, she was one of the staff at Larkin who initiated teams for girls there. O'Brien coached the first Larkin teams in field hockey (6 years), basketball (10 years), and track and field (20 years) as well as later also taking over the volleyball program. After 28 years of coaching volleyball at LHS, O'Brien's teams compiled a record of 548-280-15. At that time, she was only the 18th coach in the state to have teams with 500 victories. In 1996 Mary was inducted into the Illinois Girls Coaches Association Hall of Fame for her pioneering and successful role in coaching. She received a Meritorious Service Award from the IGCA for track and field officiating and a State Distinguished Service Award for track and field from the National Federation of Interscholastic Officials Association. O'Brien retired in 2004 after 35 consecutive years as a teacher at Larkin High School in Elgin, IL. She was inducted into the Elgin Sports Hall of Fame in 2005.