 Born in The Hague to Louis of Nassau-Beverweerd (illegitimate son of Maurice of Nassau, Prince of Orange) and his wife Isabella van Hoorn, Overkirk was baptised there on 16 December 1640.Granted the title Count of Nassau ("graaf van Nassau") by the Emperor Leopold I in 1679, he joined William III's invasion of England in 1688, and was appointed the king's Master of the Horse the following year.He resided in London, notably at Overkirk House, which later became part of 10 Downing Street.