She was the daughter of Platt Rogers Spencer, a reformer and promoter of the Spencerian Method, the widely used form of handwriting.

In 1871 she married Reuben D. Mussey, a former Union Army colonel, who was nominated but not confirmed to the grade of brevet brigadier general;[a] he was also a successful lawyer.

She obtained special consideration and was allowed to qualify for the bar by oral examination, which she passed in March, 1893.

Realizing both the scope of the task and the significance of the opportunity, Mussey sought out the assistance of a colleague and friend, Emma Gillett.

With Emma Gillett, Mussey founded the Women's Bar Association of the District of Columbia on May 19, 1917, and was elected its first President.