Here’s an ad from the June 17 1929 edition of the Toronto Daily Star for a photographer:
According to the Toronto city directories, Thornton Johnston hadn’t been at his trade for that long when this ad appeared: he is in the 1927 and 1929 Toronto city directories as a photographer, but is not in the 1924 directory.
By the late 1930s, Mr. Johnston appears to have settled into the North Toronto neighbourhood. The 1937 directory lists him at 131 Eglinton Avenue East, and the 1939 and 1944 directories list him at 836 Mt. Pleasant. (He isn’t listed as a photographer in 1939 and 1944, which leads me to wonder whether he was doing something war-related.) After the war, he seems to have settled in at 315 Roehampton Avenue, also in North Toronto; he is listed there from 1949 to at least 1964. The 1969 directory, the latest that I can access, does not list him.