Because of his inheritance, Molyneux was financially independent. Nonetheless, he held a number of official positions throughout his life. He was appointed Joint Surveyor General of the King's buildings and works in Ireland in 1684. In 1687 he invented a new type of sundial called a Sciothericum telescopicum that used special double gnomon and a telescope to measure the time of noon to within 15 seconds. He represented Dublin University in Parliament from 1692 until his death. He had also served as a commissioner of forfeited estates in 1693, resigning a few months later due to ill health.