Marjorie Susan Monaghan (born March 19, 1964) is an American former actress best known for her portrayal of Tessa Halloran (aka 'Number One') in the television series Babylon 5.
Monaghan was born in Orange County, California. She is of Irish-Celt ancestry and started acting in high school.
Monaghan has performed in a number of science fiction television roles, including starring as "JoJo" in Space Rangers and an episode of '. In 1999 she appeared in all eight episodes of Rescue 77 as a character named "Kathleen Ryan".
Monaghan is best known for her appearances in 7 episodes of Babylon 5 as a character named "Number One" (Tessa Holloran).
Monaghan started dating Grant Rosenberg in 2001 whom she later married on February 2, 2011.