While senator, Jackson served as chairman of the Committee on Expenditures in the Department of State (Sixty-second Congress).

A special election was called in 1913 to choose Rayner's successor, but Jackson chose not to become a candidate.

He served from November 29, 1912, until January 28, 1914, when an elected successor, Blair Lee I, officially qualified for the position.

Jackson later served as Maryland State treasurer from 1918 to 1920; as president of the Salisbury National Bank; and a director of the Baltimore, Chesapeake & Atlantic Railway Company.

The home he built at Salisbury in 1893, the Sen. William P. Jackson House, was listed on the National Register of Historic Places in 1976, but demolished that same year.