After graduating the University of Minnesota Law Department in June 1896, William Donohue moved to Melrose, a small town in Stearns County, where he formed a partnership with William J. Stephens, a law school classmate. He threw himself into civic activities, served as city attorney, president of the school board, state representative for one term, and county attorney for another.
He also became active in the Democratic party. He ran for attorney general in 1912, but lost in a landslide to Lyndon Smith. Four years later, he ran a spirited campaign for Congress but was beaten handily by Republican Harold Knutson.
In 1918, he moved to St. Cloud, and formed a partnership with James J. Quigley that lasted until his death on November 2, 1932, at age seventy. In an editorial the St. Cloud Times paid tribute to him:
"As an attorney he was painstaking, diligent, studious, and confident. When he appeared in court he knew the facts; and depended upon a proper presentation of the evidence, to serve his clients. He left nothing to chance, and his faithfulness in the preparation of his cases was characteristic of his life as a citizen.
"His own convictions were so well established that he never felt the necessity of fortifying them by attacks on other faiths. He had that spirit of calm tolerance that made him a delightful neighbor and a most worthy, helpful citizen.
"He filled a big place in the community, with genial fair-mindedness, good fellowship and trustworthiness."