Mary Nicholas Arnoldy (1893–1985) was a Roman Catholic Sister of St. Joseph of Concordia (Kansas), and a mathematician.

[1] Along with M. Henrietta Reilly, and Mary Domitilla Thuener, she was one of a very few women and Catholic sisters to earn a doctorate in mathematics before 1940.

In 1923–24 she broke a barrier at the Jesuits' Creighton University in Omaha, Nebraska, by becoming one of the first women to study education there.

That same graduation year, 1929, she went to Washington, DC to study at Catholic University of America, earning a master's degree in mathematics in 1930.

[9] Then in 1933 she earned a Ph.D there (minoring in physics and education) with the dissertation, The Reality of the Double Tangents of the Rational Symmetric Quartic Curve, under Aubrey Edward Landry.

[15] One of her papers was “Reality of the double tangent contact parameters of the rational symmetric quartic curve” (Mathematical Association of America, 1934).

She retired fully at age 80, moving to the Medaille Center in Salina, Kansas, that later became the site of St. John's Hospital.