Mitchell Royce first appeared in Transmetropolitan #1 - The Summer of the Year, written by Warren Ellis and illustrated by Darick Robertson. As Spider Jerusalem was significantly influenced by the late Hunter S. Thompson, Mitchell Royce was then perhaps based upon Rolling Stone publisher, Jann Wenner, who supported Thompson in a very similar way.
The Summer of the Year
As Spider Jerusalem was returning to the City to complete two unfinished books under threat of lawsuit for Driven Press, he realized that he would need a regular job in order to afford his expenses—things like journalist's insurance, an apartment, newsfeeds, and a maker with a base block. To this end, Jerusalem immediately went to Royce Mitchell, the chain-smoking city editor of The Word. While dubious at first, Royce contracted his old friend to write a weekly column.
Two issues later, as Spider Jerusalem completed his first column—a running commentary of the violent response from Civic Center to Fred Christ's efforts to secede the Transient population in the Angels 8 district to the Vilnius Colony—Royce sold newsfeed rights to Spkf, who ran it on both their public and private feedsites as an emergency report; this made Spider Jerusalem famous once again.
Right before Spider Jerusalem eventually leaves The Word, he describes Mitchell Royce in a column as "a good man"; high praise by Spider's standards. He tells Royce in person that he "only ever did right" by him."