William McArthur Sr. was an American politician from Crawford County, Pennsylvania who represented the county in the Pennsylvania State Senate for two terms from 1801 to 1808 as a Democratic-Republican.^[1]

William McArthur Sr. was born in Ireland in 1760 to John and Sarah Rule McArthur.^[2][1] The McArthur family settled in the nascent United States during the closing days of the American Revolution, originally living in York.^[1][2] William worked as a teacher and a surveyor before moving to Western Pennsylvania where he laid out the town of Meadville on May 12, 1788, on behalf of General David Mead and was appointed the State district surveyor.^[1][2] On March 12, 1800, Crawford, Mercer, Venango, Warren, and Erie counties were separated from Allegheny and were temporarily one single county, Crawford County.^[3] McArthur would be elected for two terms to the Pennsylvania Senate to represent the new Crawford County from 1801 to 1808. He was also appointed the Crawford County Prothonotary, Register, and Recorder, by Governors Thomas McKean and Simon Snyder from 1801 to 1822.^[1][2] William McArthur Sr. would die in office in 1822.^[1]

William McArthur Sr. married Rebecca McClean in 1805, the daughter of the Scots-Irish Revolutionary war Colonel Moses McClean whose brothers helped survey the Mason-Dixon line.^[2] The couple had 7 children: William McArthur, Sarah Rule McArthur, Margaret Charlesworth McArthur, Rebecca Scott McArthur, Moses McClean McArthur, William McArthur Jr., and John McArthur.^[1]