Born in Virginia, William P. Preston (1811-1880) spent much of his youth in Gettysburg, Pennsylvania. Preston studied law and by 1839 was practicing in Baltimore. Together with wife Margaret Wickham Smith (1815-1895), he maintained a residence and law office in Baltimore and a farm, Pleasant Plains, near Towson, Maryland. The couple had one daughter, May Preston McNeal (1849-1913).

On 25 October 1856 Preston presented a speech at a Democratic Party political meeting held at the farm of Robert J. T. White (1822-1889), located near the Hillsboro, Loudoun County. In his speech, Preston offers additional insight into the reason he was selected to speak to Loudoun Democrats. He notes that twenty five years earlier, at the young age of 18 he'd delivered his first public oration at the funeral of former President James Monroe (1758-1831) in the village Church at Aldie. The meeting at White's farm was held in advance of the 1856US Presidential election, held on Tuesday, 4 November 1856. In this election, Democrat James Buchanan (1791-1868) defeated Republican nominee John C. Fremont (1813-1890) and American Party nominee Millard Fillmore (1800-1874). Buchanan defeated sitting President Franklin Pierce (1804-1869), for the Democratic nomination. Speaking on Pierce's behalf. Preston urges attendees of the meeting to throw their support behind James Buchanan in the upcoming election in an effort to insure that Pierce's supporters would rally in support of to the Democratic Party's ticket.

Preston ran for Congress in Maryland's third congressional district in 1859. In the run-up to the election, Preston was attacked by a Baltimore street gang, the Plug Uglies, and received a number of head injuries. Unsuccessful in his bid for office, he contested the results of the election, but in the wake of congressional investigations of voter intimidation and other tactics used by his opponent J. Morrison Harris (1817-1898), failed to win the seat.