Rosse was the second son of Sir Laurence Parsons, 3rd Baronet, of Birr Castle, County Offaly, by his wife Anne, daughter of Wentworth Harman.
In 1792 he was raised to the Peerage of Ireland as Baron of Oxmantown, in the County of Dublin, with remainder to his nephew Sir Lawrence Parsons, 5th Baronet.
Lord Oxmantown was even further honoured in 1806 when he was created Earl of Rosse in the Irish peerage, with similar remainder as for the barony.
He died in April 1807, aged 57, when the viscountcy became extinct; he was succeeded in the barony and earldom according to the special remainder by his nephew, Lawrence.
Lady Rosse died in January 1838 and the estates were inherited by their only child, Lady Frances (1775 - 1841), wife of Robert King, 1st Viscount Lorton, whose descendants were the King-Harman family of Newcastle House, County Longford.