Roundtree attended Spelman College, graduating in 1938 and teaching for three years before Bethune selected her as one of forty women to comprise the first African American training class of WAAC officers. She served as a recruiter travelling across the South, where she endured being removed from a bus and also was forced under threat of arrest to give up her seat to a white male Marine. After the war she worked with A. Philip Randolph to make the wartime Fair Employment Practice Committee a permanent entity. During this work she met Pauli Murray who encouraged her to attend law school.
She was one of five women in her 1950 graduating class at Howard Law School; during her time she worked with Thurgood Marshall and other faculty members researching and preparing school desegregation cases which ultimately led to the 1954 Brown decision. She went into practice with Washington DC civil rights attorney Julius Winfield Robertson, and the firm took on the case of Sarah Keys vs Carolina Coach Company. Keys was a WAC private who had been forced to give up her seat much like Roundtree had, and her case challenged the right of a private bus company to impose Jim Crow laws on passengers travelling across state lines. After the case was dismissed by the DC Federal Court it was presented to the Interstate Commerce Commission which ruled in favor of the plaintiff in November 1955 based on the argument that Brown, which was decided by the US Supreme Court earlier that year, was applicable to transportation as well as education. However, it was not enforced by the ICC, and not put into practice until 1961 when Attorney General Robert Kennedy cited Keys v CCC in a Justice Department petition requesting that the ICC abide by the regulations it had approved six years earlier.
In addition to civil rights and criminal law, Roundtree also practiced family and ecumenical law as part of her ministerial duties at Allen Chapel AME. Even after retirement from her law practice in 1996 she continued to advocate for children and victims of urban violence through religious and legal organizations. She is one of the subjects interviewed in the National Visionary Leadership Project series of conversations with extraordinary African American elders.