advisor to Delta Sigma Theta sorority for 35 years. She was in contact with the Alpha Chapter of Delta Sigma Theta since 1915 when she arrived at the University of Pennsylvania. However, she needed five students to charter a chapter of the sorority, which was not possible until 1918. In March 1918, the Gamma Chapter of Delta Sigma Theta was established with Mossell as its first President. At the request of the Alpha Chapter, the four existing chapters of Delta Sigma Theta were called to convene at Howard University in December 1919. The sorority planned to host their meetings in the women's dormitory on campus until Mossell's uncle Lewis Baxter Moore offered his office as a meeting place. At this convention, the Grand Chapter of the sorority was established, taking the sorority from a loose federation of chapters to a national body. Under, Mossell's leadership the Sorority expanded to new locales in the West, the South, and further into the Midwest and Northeast. She also initiated Delta's first national program, May Week. In 1923, Mossell married Raymond Pace Alexander shortly after he was admitted to the bar, then returned with him to Philadelphia. Mossell received job offers from several Black colleges and universities, but none of them was located in Philadelphia, and she had no desire to leave her new family. So she stayed home for a year, did volunteer work, and eventually entered law school. She was the first African-American woman admitted to the University of Pennsylvania Law School. While a law student, the dean attempted to deny her participation on the law review, but her fellow students – including Philip Werner Amram, who was then editor-in-chief – insisted that she be allowed this honor, which she had earned. In 1927, she was Penn's first African-American woman graduate, and the first to be admitted to the Pennsylvania Bar. Mossell Alexander practiced law from 1927 until her retirement in 1982. Upon admission to the Bar, she joined her husband's law practice as partner, specializing in estate and family law. They both were active in civil rights law as well. In 1928 she was the first African-American woman appointed as Assistant City Solicitor for the City of Philadelphia, serving to 1930. She was reappointed from 1934–1938. From 1943–1947, she was the first woman to serve as secretary of the National Bar Association. She was appointed to the Commission on Human Relations