American lawyer, educator, and reformer who, self-tutored in the law, helped establish educational opportunities for women in that field and campaigned to improve women's legal rights.

Ellen Spencer was the daughter of Platt Rogers Spencer, reformer and promoter of the widely used system of handwriting called Spencerian penmanship. At age 12 she began assisting in his penmanship school. After his death two years later she lived with various relatives and attended Rice's Young Ladies' Seminary in Poughkeepsie, New York; Lake Erie Seminary (now College) in Painesville, Ohio; and Rockford (Illinois) Seminary (now College). In 1869 she moved to Washington, D.C., and took charge of the ladies' department of the Spencerian Business College operated by her brother Henry.

In 1871 she married Reuben D. Mussey, a former Union army general and a successful lawyer. She began working in his law office in 1876, after he suffered a serious illness, and she continued helping him in legal matters until his death in 1892. Although she had no formal training in the law and despite being refused admission to the law schools of National University and Columbian College (now George Washington University), Mussey obtained special consideration and was allowed to qualify for the bar by oral examination, which she passed in March 1893. In 1896 she was admitted to practice before the Supreme Court.

In that year Mussey began holding informal classes for three young women who wished to read law with her. After two years of these classes, and after the denial of admission to Columbian College of her students on grounds of sex, she helped establish and incorporate the Washington College of Law in 1898. From 1898 to 1913 Mussey served as dean of the college, which trained large numbers of women, as well as men, for the bar, and she also taught classes in constitutional law, contracts, wills, equity, and other topics.

Outside the college Mussey was active in numerous reforms. In 1894 she joined the campaign for improved married women's property rights (Married Women's Property Acts) legislation, and the following year she successfully urged the creation of a legislative committee by the District of Columbia Federation of Women's Clubs. As chairman of the committee she led the campaign that resulted in the Married Woman's Act, passed by Congress in June 1896, which equalized the status of married women with respect to property and guardianship of children. She was instrumental in securing a congressional appropriation in 1898 for the establishment of public kindergartens in the District, and she played a major role in the establishment of a juvenile court system.

Mussey served on the District of Columbia board of education (1906–12), as vice president general of the Daughters of the American Revolution (1907–09), and as editor of the DAR's American Monthly Magazine (1911–12). Her legal practice included serving as counsel to the American Red Cross and for 25 years to the legations of Sweden and Norway.

Mussey suffered a stroke in March 1913, and later in the year she retired as dean of the Washington College of Law. She later resumed limited activity and in 1917 became chairman of the committee on the legal status of women of the National Council of Women. She drafted and, with Maud Wood Park, she helped secure passage in 1922 of the Cable Act, which ended the automatic loss of citizenship for women who married foreign nationals.