1792, "person learned in public law or the law of nations," from public (adj.) + -ist. Also from 1795 in English as "writer on current topics," from French publiciste; in either case a hybrid.
Then crept in the "loose" usage. Anybody who wrote or spoke about public affairs came to be dubbed a publicist. It was only a question of time when the dam would give way and the word flow in all directions and be made to cover every kind of talent, or lack of it. ["The Nation," Nov. 22, 1917]
Meaning "press agent" is from 1925 (publicity agent attested by 1900); publicitor also was tried in this sense.