Interestingly the term is entirely missing from Superman's wiki-page.
I was wondering who coined this famous phrase?
Per this article in the New York Times, the phrase was invented by the writers of the 1940s radio serial. No specific writer is credited although Siegel and Schuster certainly wrote the original version ("truth and justice") since it's in the earliest comics:
Where did that specific phrase come from? According to Mark Waid, a former DC Comics editor, it first turned up on the innovative "Adventures of Superman" radio series, which ran, off and on, from 1940 to 1951. It was the radio show, not the comic book, that introduced many facets of the Superman myth.
Since Superman was a work in progress, it makes sense that the preamble was a work in progress, too. Fans first heard "Up in the sky! Look!" rather than the other way around. Those who did look thought they saw not a bird but "a giant bird." At one point the Fleischer cartoons even scrapped the whole "speeding bullet" business in favor of more weather-oriented metaphors: "Faster than a streak of lightning! More powerful than the pounding surf! Mightier than a roaring hurricane!"
Then, in autumn 1942, fans of the radio show became the first to hear about Superman's battle for "truth, justice and the American way."
At that time the war was not going well. Field Marshal Erwin Rommel was sweeping across Africa, and the German Army was driving toward Stalingrad. The Japanese had been turned back at Midway but they were still invading Pacific islands. Americans were all fighting for the American way. Why shouldn't Superman?
As the war turned in our favor, though, the additional phrase didn't seem as necessary. By 1944 it was gone, and for the remainder of the radio show, Superman devoted himself to the fight for tolerance.
It took the paranoia and patriotism of the Cold War era to bring back "the American way" - this time in the "Adventures of Superman" TV series, which ran from 1952 to 1958. Every week, young baby boomers were greeted with the phrase as they sat down to watch the Man of Steel combat crooks and Communist spies.
You can hear the earlier version in this 1941 animation:
First of all, the motto appeared for the first time in The Adventures of Superman radio series, as we can read in the book The Ages of Superman: Essays on the Man of Steel in Changing Times (page 225):
A 1942 episode of the Adventures of Superman radio series was the first appearance of the “truth, justice, and the American way” phrase, but it became cemented in the Baby Boomer psyche as part of the weekly introduction of the Adventures of Superman television series that ran from 1952 to 1958.
The fact is confirmed also by this article which reports Mark Waid interview. I didn't find any direct attribution of the quote, but we can assume it has been reformulated (from the previous motto "truth and justice") by either B.P. Freeman or Jack Johnstone, script writer for the radio series.
The Smithsonian Institute in Washington DC has a museum of American history. In the 1980s, I visited and was floored by a small sign in a small exhibit on the postwar years that featured Superman. The sign said the slogan had been "truth, justice, and tolerance" originally, and yes, that it was changed in the Cold War years. I trust the Smithsonian though I don't know how to fact check an exhibit from 30-some years ago.