Technical Communication became a profession, in the sense of having professional associations, in the early 1950s. As a discipline, we draw on many other areas. The history of rhetoric, for example, dates back to the classical Greeks.
Sources and Information
An article in the history of TC from 1850 to 2000 by Earl E. McDowell (PDF).
A link to the book Three Keys to the Past: The History of Technical Communication, by Kynell and Moran.