Risteau was born in Towson, Maryland on April 24, 1890. She graduated from Towson High School in 1907 and completed a special advanced course of study in Mathematics at Johns Hopkins University in 1917 before serving as a schoolteacher. In 1938, Risteau received her L.L.B. Degree from the University of Baltimore.
In 1921, Risteau, a Democrat who at the time lived in Jarrettsville, became the first woman elected to the Maryland House of Delegates and served in four sessions, followed by an election to the State Senate in 1935. She served on several committees, including the Committee on Education and the Committee on Agriculture, during her time in the General Assembly, and she was a strong sponsor of women's rights. Concurrent to her role as a legislator, she served as the first woman on the Maryland State Board of Education for 16 years. She was appointed to this position by Governor Albert Ritchie. Risteau was also a clerk of the Circuit Court for Harford County; she was appointed in 1938 and served for one year.
In 1951, she was elected one final time to the House of Delegates, where she served for another four years. She died on July 24, 1978, at the age of 88.
- Ruth, Janice. "Mary E. W. Risteau Collection, 1845–1978 Maryland Historical Society". mdhs.org. Maryland Historical Society. Retrieved 22 March 2015.
- "Women Wielding Power-Maryland". nwhm.org. Retrieved 8 September 2015.
- Maryland Commission for Women. "Mary Riseau". msa.maryland.gov. Maryland State Archives. Retrieved 22 March 2015.