Soule was born in Kennebunk, York County, Maine and went to public school in California, where he had moved as a child. He learned the machinist's trade and worked for a time at a newspaper printing plant. In 1859 he began the study of law and was admitted to the Bar of California in 1861. Initially unsuccessful in business, he subsequently moved to San Francisco and became a successful attorney.
Becoming interested in politics, he was a delegate to the California State Democratic Conventions of 1871, 1873, and 1877. At the 1880 Democratic National Convention, he was chairman of the California delegation and on behalf of the State of California, seconded the nomination of Winfield S. Hancock.
In 1878 he was elected as a Democrat to represent California in the Forty-sixth Congress and reelected to the Forty-seventh Congress. He served from March 4, 1879, to March 4, 1883. He ran for re-election in 1882, lost, but, when the original choice declined, was elected to fill the vacancy in the Forty-eighth Congress and served from January 31, 1884, to March 3, 1885. He was succeeded as Democratic nominee by William W. Morrow, who was elected over Soule, running as an independent, in 1884.
As a U. S. Congressman, Soule was chairman of the Committee on War Claims (Forty-seventh Congress) and the powerful Committee on Ways and Means (_Forty-eighth Congress_) and is credited with having written the _Anti-Chinese Immigration Act of 1882_, the first legislation enacted by the United States specifically restricting immigration. He was also an instrumental figure in the enactment of the McKinley Tariff Act of 1890, considered by its beneficiaries to be the pinnacle of protectionist legislation in American history. This act significantly reduced customs revenues, necessitating a new source of revenue. In that same year, Congress passed the Sherman Anti-trust Act, which declared illegal all combinations "in restraint of trade" and monopolization.
Soule ran twice as a Democrat for a seat in the United States Senate, for elections to the Forty-ninth and Fiftieth Congresses, losing to Republican George Hearst and Democrat George C. Perkins, respectively. He died of neuralgia in San Francisco, while arguing a cause before the State Supreme Court, leaving a wife, two sons, and a daughter. He is buried in Odd Fellows' Cemetery in Columbia.
Following unsuccessful prospecting at the Yellow Dog Mine in the Kern River Valley, Soule purchased the property in July 1882 and established a ranch as well as a toll road to the Kern River Valley, which is called _"The Soule Grade."_