He succeeded to the peerage following the death of his great-nephew William in August 1648, to whom he was served heir on 20 March 1649.
After his succession, he became embroiled in a legal dispute with the Countess of Eglinton (the widow of his nephew James) in relation to the charter-chest of the House of Ross, which she apparently declined to produce.
Prior to his succession to the peerage, he was known generally as William Ross of Torphin, or Sir William Ross of Muriston, or Newriston.
In 1649 he was also a commissioner for the plantation of kirks (intended to provide for the regular and permanent endowment of the ministers of the Church of Scotland).
He married secondly Margaret, the eldest daughter of Sir James Forrester of Torwoodhead.