Charles Mason was assistant to the Astronomer Royal, James Bradley, at the Royal Observatory, Greenwich, from 1756 to 1760. As an experienced observer he was chosen by the Royal Society in 1760 to observe the transit of Venus on 6 June 1761 at Bengkulu (then called Bencoolen), a small port on the south-west coast of Sumatra. He was to be assisted by the surveyor Jeremiah Dixon*. Within hours after sailing on the Seahorse from Portsmouth in December 1760 their ship was attacked by a French frigate (the two countries were at war at the time) and had to return to port to refit. Mason wrote to the Royal Society refusing to sail again, but changed his mind when the society threatened court action. They eventually reached the Cape of Good Hope on 27 April 1761, probably too late to reach their destination in time. Furthermore, Mason learned that Bengkulu had been taken by the French and decided to observe the transit from the Cape. This turned out to be a fortunate turn of events, as a second British expedition led by the astronomer Nevil Maskelyne sent to observe from the island of St Helena was clouded out.
Mason and Dixon erected a small observatory building and set up their equipment at Concordia Gardens in Cape Town, a public pleasure garden near Hope Street and Bouquet Street, behind the present St Mary's Cathedral. They had two reflecting telescopes of 600 mm focal length, a quadrant of 300 mm radius, and an astronomical clock. From 4 May to 27 September they observed the meridian passage of a number of bright stars to correct their clock, the zenith distances of the same stars to determine the lattitude of their observatory, eclipses by Jupiter of its larger satelites to fix their longitude by comparison with similar observations in England, a total eclipse of the moon on 18 May, and of course the transit of Venus on 6 June. They measured the position of Venus against the sun's disc during the transit, as well as the time of intenal contact with the sun's limb towards the end. Despite some interference from clouds their observations were successful and accurate. An account of the work, "Observations made at the Cape of Good Hope by Mr Charles Mason and Mr Dixon..." was published in the Philosophical Transactions of the Royal Society of London for 1761 (Vol. 52, pp. 378-394), with a brief appendix by Mason, "Latitude of the observatory at the Cape of Good Hope, reduced from observations of different stars" (p. 395). By comparing their observations with those of fifteen observatories in the northern hemisphere, J. Short derived a mean solar parallax of 8,65 seconds of arc (Philosophical Transactions, 1762, Vol. 52, pp. 611-628). The modern value is 8,79 seconds of arc. An exhaustive review of all the observations during this transit, by 176 observers at 117 stations, was published in 1891 by Simon Newcomb*. The weights he assigned to Mason and Dixon's observations were among the highest that he allotted.
Sailing from the Cape on 3 October 1761 Mason and Dixon travelled to St Helena, where Mason assisted Maskelyne for two months in collecting tidal data and astronomical researches. As a result of their success the two observers were chosen in 1763 to survey the boundary line between Maryland and Pennsylvania in the United States. The line was to run exactly east-west (along latitude latitude 39 degrees 43' 17.6" N) from a point south of Philadelphia. In November 1767, having surveyed 392 km with only some 58 km remaining, further progress towards the west was stopped as a result of opposition by the local natives. This so-called "Mason and Dixon line" became famous as the supposed boundary between slave-holding and non-slave-holding states. The work was written up under both their names as Field notes and astronomical observations... made in their survey of the boundary lines between the provinces of Pennsylvania, Delaware, and Maryland, 1763-1768 (n.d.). They also measured an arc of meridian for the Royal Society, at a mean latitude of 39 degrees on the Delmarva peninsula in Maryland, using wooden rods and without any triangulation. Despite taking great care the results were not completely satisfactory. They reported this project in "Observations for determining the length of a degree of latitude in the provinces of Maryland and Pennsylvania, in North America", published in the Philosophical Transactions of the Royal Society in 1768. Some gravity observations were also made, using a pendulum clock. Their work was finally completed in June 1768. Both men were elected as members of the American Philosophical Society held at Philadelphia for Promoting Useful Knowledge.
Mason was employed by the Royal Society to observe the next transit of Venus on 3 June 1769 (not visible from South Africa) in Ireland, and for other work. His transit observations were published as "Astronomical observations made at Cavan, near Strabane, in the County of Donegal, Ireland" in the Philosophical Transactions for 1770. In 1773 he was appointed by the Royal Society to measure the effect of a mountain on the deflection of a plumb line as a result of its gravitational attraction. He recommended the hill Schiehallion, in Perthshire, Scotland, for the experiment, but declined to do the field work. His other work included completing the computations for a catalogue of 387 stars, which was later published in the Nautical Almanac, and compiling improved lunar tables at the request of the Board of Longitude. These tables were published in the Nautical Almanac from 1787, to enable seafarers to determine their longitude at sea from observed distances between stars and the moon. In 1786 Mason returned to Philadelphia, where he died later that year.