Mary Rice attended Swarthmore College in Pennsylvania, where her mother's brother Frank Aydelotte was president, and graduated with highest honors.
As a housewife, Mrs. Marshall became politically active in the League of Women Voters, then the local Democratic Party (which she found conservative but not in lockstep with the Byrd Organization).
Marshall consistently opposed the Byrd Organization's Massive Resistance against the Supreme Court's school desegregation decisions in Brown v. Board of Education, and later commented that she was initially one of 17 "liberals" out of 70 local Democratic Committee members.
Arlington voluntarily desegregated in February 1959 after Governor J. Lindsay Almond broke with the Byrd Organization and acceded to decisions by a three-judge federal panel and the Virginia Supreme Court issued on January 19, 1959.
As a result of the U.S. Supreme Court's decision striking down the previous reapportionment in Davis v. Mann, Arlington received another seat in the General Assembly.
Virginia Democrats experienced a backlash against the sales tax, as well as a speech by President Richard M. Nixon, who urged people to vote Republican in the New Jersey gubernatorial race, but which had broader effects.
Courteous Clive L. DuVal II of Fairfax and Byrd Organization stalwart James M. Thomson of Alexandria were the only Democrats elected to the General Assembly from northern Virginia that year.
She sponsored landmark legislation modernizing state policies concerning the elderly (including equal treatment of widows in inheritance and pension taxes, and rights of nursing home patients), and a variety of issues relating to women and children, mentally retarded persons and libraries.
[14] Marshall died at Arlington Hospital on October 9, 1992, aged 71, never regaining consciousness after suffering a head injury during a fall at her home while clearing a table.