William James Ellis was born in St. John's on 9 July, 1857 and received his early education at St. Patrick's Riverhead School. After apprenticing as a stonemason with his father, David, by 1890 along with his brother, John, he had established his own construction business, which in the early 1890s built the main tunnel for a new sewerage system in St. John's. The rebuilding boom after the fire on 8 July, 1892, that destroyed much of St. John's, proved a great boon to the Ellis contractors, who put up many of the town's new buildings. After 1892 their work included the Total Abstinence and Mechanic's Halls, the Commercial Chambers on Water Street, the King George V Institute, the tower and spire of St. Patrick's Roman Catholic Church, the restoration of the walls and towers of the Basilica, an extension to Littledale Academy, and the club house for Bally Hally Golf Club. He also owned and operated a large quarry on the Southside in St. John's and, according to the Newfoundland Quarterly in 1910, he was "one of the largest employers of labor" in St. John's.

Ellis was active in several Roman Catholic social organizations such as the Total Abstinence Society and was its President from 1910 to 1926, the Benevolent Irish Society, the St. Vincent de Paul Society, and the Knights of Columbus. In the 19 June 1902 municipal election his popularity and prominence in the community saw him poll the second highest vote for one of the six councillor positions. Four years later he received the highest vote for councillor and became Deputy Mayor under Mayor Michael P. Gibbs. Elected in 1904 as a Liberal to represent the District of Ferryland, he won re-election in 1908, but in the 1909 and 1913 general elections he unsuccessfully headed the three-member Liberal ticket to St. John's West to unseat the People's Party candidates led by Premier Edward P. Morris.

In the 27 June 1910 municipal election, Ellis won the mayoralty of St. John's by acclamation. In the platform he presented to voters, Ellis promised more water and sewer extensions to eliminate the sanitary night cart service, the construction of inexpensive housing for workingmen, the creation of a municipal electric lighting system, the establishment of city fish markets, the reduction of taxation, the paving of Duckworth and New Gower Streets, and the purchase of a building to house Council officials and records. During his term as mayor from 1910 to 1914, Ellis effected some improvements for instance, water and sewer extensions and the opening in 1911 of a city-owned building on Duckworth Street but his ability to act on many promises was seriously handicapped by limited revenues and a restricted city act passed in 1902 by the legislature.

Public disenchantment with Council led in December 1913 to the formation of a civic reform movement under the leadership of Water Street merchant William Gilbert Gosling. In early 1914 Gosling convinced Premier Morris to replace the elected Council by an appointed Commission, which would have responsibility for governing St. John's and drafting a new municipal act. This action had followed an unsuccessful attempt in January 1914 by Ellis to circumvent Gosling's movement with reform proposals of his own, but his initiative failed in the face of Gosling's broader and more popularly based plans. Although appointed to the Municipal Commission by Morris, Ellis refused to take his place when on 2 July 1914 the other Commissioners voted for their own chairman, Gosling. Ellis had accepted appointment to the Commission only on the understanding from the government that he would be appointed chairman, a position he claimed to be rightfully his because of his long record of municipal service.

Ellis remained active in colonial politics and accepted appointment as a Legislative Councillor and a minister without portfolio in the National Government Morris formed in July 1917. He subsequently served also in the same capacity in the National Government of William Lloyd (January 1918 - May 1919) and in the People's Party Government of Michael Cashin (May-November 1919). During the First World War, Ellis was also a member of the Newfoundland Patriotic Association, which from 1914 to 1917 administered the colony's war effort. He died on 19 April 1926 in New York City while returning to Newfoundland from California where he had gone for the winter for health reasons.