[1] Cooney studied law with the firm of Bacon & Sawin and was admitted to the bar on November 1, 1868.
In 1881 he was the Republican nominee for Natick's seat in the Massachusetts House of Representatives, but lost the election by 3 votes.
In 1884, Cooney was appointed an assistant district attorney for Middlesex County, Massachusetts.
After leaving the DA's office, Cooney focused on defending corporations, notably defending the West End Street Railway and Boston Elevated Railway in accident and land damage cases.
In January 1903, the Sarah Allen Cooney Memorial Church was dedicated in Natick by Edward Everett Hale and Samuel A.