He entered the Inner Temple and became a barrister in the Oxford Circuit, but never pursued his career with undue enthusiasm and ceased taking cases by 1907.

In the words of John Ramsbottom, Rea was "active in his leisure" and devoted much of his time to natural history, having joined his local Worcestershire Naturalists' Club as a schoolboy (he was president of the club in its centenary year, at the time of his death).

Rea's special interest was in fungi and in 1896 he was one of the founder members of the British Mycological Society.

Rea was a keen field mycologist, cutting a distinctive figure at forays in Panama hat, white waistcoat, knickerbockers, and monocle.

Carleton Rea wrote a series of papers on British fungi, particularly agarics, and described a number of new species.