James O'Neill was born in Duanesburg, New York, in 1824. Of Irish descent, his father was a shopkeeper in Schenectady County, New York, where the younger James was born. After receiving his education he worked in his father store for a time before moving to Albany where he worked as a store clerk.
James was educated in the local public schools as well as the academies in Gallupville and Albany.
A New York native, he was mayor of Portland, , and an Indian Agent in territories. He later served in the legislatures of the Washington Territory and the State of Washington. He was the oldest of five children born to James E. O'Neill and the former Elizabeth Marsh.
From 1849 to 1851 he was in business for himself in Albany, and in 1851 he moved to New York City and continued his business until 1853. Caroline would die in 1871. In 1853, O'Neill moved to the Territory, traveling by ship to Panama where he then traveled by land across the Isthmus.
He then continued by ship to In , he settled in City where he worked as an agent for Wells Fargo & Company until 1857. A few years after arriving he moved across the Willamette River to Portland. A Whig Party supporter, he attended that party’s first convention in 1855 in Corvallis.
On April 7, 1856, O'Neill was elected as the seventh mayor of Portland, replacing George W. Vaughn. He was re-elected the next year, becoming the city’s first two-term mayor, but was replaced on November 4, 1857, by William S. Ladd before the end of his second term. In 1858, O'Neill returned to New York briefly, and then returned to Portland as a merchant, remaining until 1861.
In 1863, he was working as an interpreter for the Office of Indian Affairs in the Idaho Territory. O'Neill worked out of Lapwai and in 1865 was working as an Indian agent for the Nez Perce Agency in Idaho. He continued in that position through 1867.
He then moved to Cheweela on the Colville Indian Reservation and continued working for the Office of Indian Affairs. From 1868 until 1878 he was back in New York. O'Neill returned to Washington in 1878 and worked as a farmer for the Coeur d'Alene Tribe.
He kept that position until resigning in 1887. During this time he also served as postmaster for Chewelah, while in 1881 he served in the Washington Territorial House of Representatives of the Washington Territory representing Spokane and Stevens counties. While in the House he worked to pass the bill incorporating the city of Spokane Falls (now Spokane) in 1881.
In 1888, O'Neill was elected as auditor for Stevens County, serving for two years. James O'Neill was elected to the Washington State Senate in 1889 when Washington became a state. He represented the Second District, made up of Spokane and Stevens counties and served as chairman of the mines committee and of the Indian Affairs committee.
In 1892, he served as a delegate to the Seattle Convention. He died on July 21, 1901 in Spokane, Washington and was buried in Fairmount Cemetery.
He switched political allegiance to the Republican Party when that party was formed.
