Cadogan, William (1601–61), soldier and MP, was born 5 February 1601 at Cardiff, Glamorganshire, Wales, eldest son of Henry Cadogan of Trostry Fach, Monmouthshire, and Catherine Cadogan (née Stradling). As a captain of horse, he became private secretary to Thomas Wentworth (qv) in Ireland (1633), and also served as secretary to Christopher Wandesforde (qv), master of the rolls; as MP for Monaghan borough (1639–49), he sat on a committee to impeach Wentworth's other associates (1641). When rebellion broke out in October 1641, he was commissioned in Tichborne's regiment, commanded the garrison at Trim, Co. Meath, and took part in the first siege of Drogheda. On surrendering Trim (1649) to Lord Inchiquin (qv), he chose to join the parliamentary army in Dublin, became a major in Foulkes's regiment, and resumed governorship of the castle and borough of Trim (1649–61).
He served as commissioner for the survey of Antrim, Monaghan, Louth, and Meath; acquired an estate at Liscartan, Co. Meath, and a house in St George's Lane, Dublin; represented Meath and Louth in the 1654 union parliament; and was one of the representatives for Meath in the Dublin convention of early 1660. He was portreeve of Navan in 1660. Cadogan died 14 March 1661 and was buried five days later in Christ Church, Dublin. He married first Elizabeth Thring (d. 25 January 1641) of Drogheda, Co. Louth; secondly Elizabeth Roberts (d. 20 February 1665; also buried in Christ Church) of Caernarfon, Wales. Their only son, Henry, married Bridget, daughter of Sir Hardress Waller (qv).