Born in Charleston, West Virginia, Hallanan received a Bachelor of Arts from Morris Harvey College (now the University of Charleston) in 1946 and a Juris Doctor from West Virginia University College of Law in 1951.
She was a member of the West Virginia State Board of Education in Charleston from 1955 to 1957.
She served in the West Virginia House of Delegates from 1957 to 1958, and was then appointed as an assistant commissioner of public instruction from 1958 to 1959, and then as a judge on the Juvenile Court, Kanawha County, West Virginia, from 1959 to 1961.
[1] On November 8, 1983, Hallanan was nominated by President Ronald Reagan to a seat on the United States District Court for the Southern District of West Virginia vacated by Judge Dennis Raymond Knapp.
She assumed senior status on December 1, 1996, serving in that capacity until her death, in Charleston, on June 8, 2004.