I don't know if that is true or not, but I believe that Earl Warren and Brennan also had never been judges prior to their appointments. More food for thought.
Earl Warren had been Attorney General of California for two terms, and before then Distict Attorney of Contra Costa County, California, for several terms, before being elected Governor for three terms. He had more than adequate senior legal administrative experience for a Supreme Court appointment. Prior judicial experience is not necessary provided there is adequate other experience, and two terms as a state Attorney General is that. I believe William O. Douglas had been Chairman of the Securities & Exchange Commission, which is a comparably important federal legal administrative position.