Hastie received a BA from Amherst, where he finished first in his class, and then received a law degree (1930) from Harvard, becoming the second African American to serve on the Harvard Law Review. He then taught at Howard University Law School, where he worked with his friend, Charles Hamilton Houston, and his student, Thurgood Marshall, among others, to develop legal challenges to segregation. In private practice, as part of the law firm Houston, Houston, and Hastie, he argued a number of civil rights cases.
In 1933, Hastie was appointed Assistant Solicitor in the Department of the Interior by President Franklin Roosevelt, and in 1937 Roosevelt appointed him judge of the Federal District Court in the Virgin Islands, making him the country's first African-American federal magistrate. He left that position in 1939 to become Dean of Howard Law School. In 1941 Hastie became an aide to Secretary of War Henry L. Stimson, and worked to reform the military's segregationist policies. But Hastie resigned from that position in 1943 to protest the military's entrenched “reactionary policies and discriminatory practices.” That year he was awarded the Springarn Medal “for his distinguished career as jurist and as an uncompromising champion of equal justice.”
In 1946, Hastie became the first African-American governor of the Virgin Islands, and in 1949 President Truman appointed him judge of the Third United States Circuit Court of Appeals, making him the first African American to be appointed as a federal circuit judge.