Hays was educated at Columbia University where he received his B.A. degree in 1902 and his law degree in 1905.
Hays established a lucrative legal practice, representing numerous corporate institutions, public figures, and wealthy individuals. During the first years of World War I, he lived in London where he represented American shipping interests.
Devoted to the cause of civil liberty, Hays served as general counsel of the American Civil Liberties Union (ACLU) from 1912 and was associated with that pioneer organization all his life. He was defense counsel in most of the celebrated civil liberties cases of his time. These included the 1925 Scopes “monkey trial” in Tennessee (with Clarence Darrow), the Sweet housing segregation case in Detroit in 1926 (also with Darrow); the obscenity trial of H. L. Mencken and the American Mercury magazine in Boston in 1926; the Sacco and Vanzctti anarchist case in Boston in 1927; the case of communists accused of burning the Reichstag in Berlin in 1933; and the Scottsboro case in Alabama in 1931.
Hays was also active in politics, first as a supporter of Theodore Roosevelt, then as a founder of the Farmer-Labor parly in 1919, and as New York State chairman of the Progressive (La Follette) party in 1924.
Member New York State and New York County bar associations, Association of Bar of City of New York.
Married Blanche Marks, 1908.; married second, Aline Davis Fleisher, June 12, 1924.