Jenckes was born in 1656, in Pawtucket, Rhode Island, the son of Joseph Jenckes. His father, founder of what is now the city of Pawtucket in Rhode Island, married Esther, the daughter of William Ballard of Lynn, Massachusetts and Joseph, the eldest of their ten children, was born and grew to manhood in Pawtucket.
Jenckes adopted the profession of land-surveying, his ability soon making him a distinguished man in the colony. For several years (1691, 1698, 1700-1708) he was a deputy to the General Assembly, acting for two years of that time as speaker (1698-1699, 1707-1708). Later (1708-1712), he held the more responsible post of assistant. In 1715 he was elected deputy governor and continued as such for twelve years. His competence as a surveyor proved of great value to his fellow colonists. In 1710 he was empowered to treat with Colonel Dudley on the matter of the Massachusetts boundaries, and on the strength of his success in this affair, he was in 1720 appointed to act, together with the colony's agent in England, to settle the much more difficult question of the Connecticut boundaries. His efforts in London were entirely satisfactory to his constituents, but the boundary disagreement dragged on for years. On the death of Governor Cranston, who had held office for thirty successive terms, Jenckes was chosen in 1727 to succeed him. He continued as governor for five years, announcing in 1731 that he would refuse reelection. He would scarcely have obtained it in any case. The finances of Rhode Island had become involved by the introduction of paper money, and when a bill to emit sixty thousand pounds additional currency was passed by the General Assembly, the Governor noted his dissent on the document. This veto was declared ineffectual by the papermoney party which overruled his action. After hot dispute both the Governor and the General Assembly appealed to the home government. The latter's reply, which found that within limits neither the governor nor the Crown could change such an act of the Assembly, confirmed Jenckes's opponents and ended his political career. During his life Jenckes had given much attention to military as well as political affairs and progressed through the various ranks from lieutenant to colonel. From 1707 to 1711 he acted as Major of the Main Land. Jenckes died on June 15, 1740, in Providence, Rhode Island.
Jenckes was a man of extraordinary size and strength. Seven feet two inches tall and splendidly proportioned, he was an imposing figure in any gathering.
Jenckes was twice married. His first wife was Martha Brown, a granddaughter of Chad Brown, the associate of Roger Williams. She died leaving nine children. In 1727 he married a widow, Alice (Smith) Dexter, who died childless in 1736.
