Sadie Mossell was born in Philadelphia, Pennsylvania, to a prominent black Philadelphia family. Her father, Aaron Mossell, was the first African American to receive a law degree from the University of Pennsylvania. Her grandfather, Benjamin Tucker Tanner Tanner, edited the first black scholarly journal in the United States, the A.M.E. Church Review.
Mossell received her doctorate from the University of Pennsylvania in 1921. She worked as an actuary in North Carolina, then left to marry Raymond Pace Alexander, a graduate of Harvard Law School. With her husband's encouragement, she returned to the University of Pennsylvania, earning her law degree in 1927. The two entered law practice together. Their civil rights work began in 1935 when husband and wife fought to end racial segregation in Philadelphia The Alexanders visited segregated city theaters hotels and restaurants to demand rightful admittance under law and agitated for ...