Nicholson was Director-General of the Nature Conservancy, 1952-1966. The Nature Conservancy Council replaced the Nature Conservancy in 1973.

See MS. 15041/1695 for correspondence concerning Nicholson's employment at the Nature Conservancy, including his transfer from the Lord President's Office. See MSS. 15041/291-298 for papers relating to conservation of the Camargue, France, with which Nicholson was involved partly in his capacity as Director-General.