In 1884 Douglas was elected as a Democrat to the Massachusetts House of Representatives, serving two terms. He then served one term in the state senate in 1887, and was elected mayor of Brockton for a single term in 1890. He was regularly active in the party organization, attending most of the national conventions between 1884 and 1904. He was generally a supporter of improved labor practices, supporting the creation of a labor mediation board, and advocating for the weekly pay of workers.