The family came originally from Greystock, in Cumberland, but this estate was granted to one Thomas Blencowe in the time of Henry VI.

Fifth in descent from him was Thomas, father of John Blencowe, who married as his second wife Anne, daughter of the Rev.

He was entered a student of the Inner Temple in 1663, called to the bar 1673, elected a master of the bench in 1687, received the degree of serjeant-at-law 11 April 1689, and represented Brackley in Northamptonshire for five years in the parliament of 1690, being a firm adherent of the government.

To this marriage Blencowe in part owed his advancement ; for when the deanery (or bishopric, according to Granger) of Hereford was offered to Dr. Wallis he declined it, and asked a favour for his son-in-law, saying, 'I have a son-in-law, Mr. Serjeant Blencowe, of the Inner Temple, a member of parliament, an able lawyer, and not inferior to many of those on the bench, of a good life and great integrity, cordial to the government, and serviceable to it.'

Before his death his faculties had decayed; he conceived he had discovered the longitude, and employed his son William in copying his writings to lay before parliament.