On 18 July 1765 the Board of Longitude (effectively led by Nevil Maskelyne) appointed Dunthorne as the first "Comparer of the Ephemeris and Corrector of the Proofs" for the (then still future) Nautical Almanac and Astronomical Ephemeris. The first issue appeared with data for the year 1767, breaking new ground in providing computational tools to enable mariners to use lunar observations to find their longitude at sea. Dunthorne worked as sole comparer for the first three issues, with data for 1767–69, and afterward continued as one of several comparers until the issue for 1776.