William "Will" Mailly (1871–1912) was an American socialist political functionary, journalist, and trade union activist. He is best remembered as the second National Executive Secretary of the Socialist Party of America and as the first managing editor of the socialist daily newspaper, the New York Call.
William Mailly was born November 22, 1871 in Pittsburgh, Pennsylvania. Mailly's parents emigrated to Liverpool, England when the boy was 2, and so the American-born Mailly was raised in English schools — an extremely rare pattern of emigration for an American radical activist. The young Will worked in Liverpool from a very early age as an errand boy.
Mailly returned to the United States in July 1889, working briefly in a brickyard and on a railway before moving to Alabama in 1890 to take up work as a coal miner. Mailly became involved in the trade union movement as a member of the United Mine Workers of America in 1893.
Mailly took part in a coal strike in 1894, activity which resulted in his blacklisting from the Alabama coal mines.
Following his blacklisting from the mines, Mailly began to work as an organizer on behalf of the UMWA and the Socialist Party of America.
Mailly was first a member of the People's Party, serving as a delegate to the Alabama state convention of that party and to its national convention in 1894.
Mailly turned his hand to labor journalism, taking a job as Associate Editor of the Birmingham Labor Advocate in May 1895, and serving at that post for about a year. Thereafter he moved to Nashville, Tennessee. In 1896, he joined the Socialist Labor Party of America (SLP), but his stay in that organization proved to be short-lived, as he left the SLP in July of the subsequent year to join the Social Democracy of America, an organization which included Victor L. Berger and Eugene V. Debs. Mailly organized a branch of the Social Democracy in July 1897 and was a delegate from Alabama to the 1898 Convention of this organization.