The Shadow radio was on the air from September 26, 1937 until the final episode on December 26, 1954. During that time, five men played the part of Lamont Cranston/The Shadow. This brief article will give the names and dates for each of these men. I plan on writing more about each of these actors in the future!
Orson Welles portrayed The Shadow in season 1 (1937 - 1938), for a total of 52 episodes.
William Johnstone took over the role for seasons 2 through 6 (1938 - 1943). (Shadow Comics #23 included some information on Johnstone which you can read here.)
For season 7 (1943 - 1944) it was Bret Morrison in the role of Lamont Cranston/The Shadow.
John Archer was The Shadow for one season. He was in season 8 from 1944 to 1945.
Stephen Courtleigh portrayed The Shadow for the first eight episodes* of season 9 (1945 - 1946). His last episode as The Shadow would have been "I Must Never Sleep Again" which aired on October 28, 1945.
Bret Morrison took over from Stephen Courtleigh beginning with the ninth episode of season 9 ("Spotlight on the Duchess" which aired on November 4, 1945). He stayed in the role until the end of The Shadow radio show in season 18 (1954).
*Information from Grams, Jr, Martin, 2011, The Shadow The History and Mystery of the Radio Program, 1930 - 1954, pp 258-259, OTR Publishing