1915, "one who writes serially for publication in a newspaper or magazine," from column in the newspaper sense + -ist.
The successful Columnist puts his own personality into his column. It is not a case of impersonal jesting and the heaping up of cold, blue-lit diamonds of wit. The reader likes the column because it reveals a daily insight into another man's soul—and he finds this other soul likeable. [C.L. Edson, "The Gentle Art of Columning," 1920]