While clearly identified on his election return as ‘Sir William Selby of Shortflatt’, Selby was one of three namesakes active in the administration of north-eastern England under the early Stuarts. Sir William Selby I*, a distant cousin, inherited large estates in north Durham, acquired property in Kent and died in 1612. The latter’s nephew, Sir William Selby†, was usually referred to as ‘Sir William Selby junior’ before his uncle’s death. After moving to Kent in 1612, he gradually disappeared from Northern commissions, in which he was generally referred to as plain ‘Sir William Selby’. Thereafter, this Member, who was knighted in 1613 and does not seem to have held local office before acquiring Shortflatt in 1611, was referred to as ‘Sir William Selby junior’.
As a newcomer among Northumberland landowners, Selby was an unusual choice for knight of the shire, a position habitually shared among the senior gentry. He was returned in May 1614 after the Commons unseated his elder brother, Sir George, who was deemed ineligible because he was sheriff of Durham. Sir William’s election, managed by another relation, Northumberland’s sheriff Sir Ralph Selby, was presumably intended to convey the freeholders’ displeasure at the rejection of their earlier choice.
In 1625 Selby inherited his brother’s lands in co. Durham, which included one of the largest coal-mining businesses on the Tyne, but disputes with Sir George’s widow over legacies burdened him with £11,000 worth of debt, which he cleared by selling part of the estate.