According to Resolution No. 46, passed in spring 1965 by the National Executive Committee, "A Legionnaire is considered to be in uniform if wearing an offical American Legion Cap."
But this was not always the case. As far back as 1922, a resolution was put forward at the national convention in New Orleans on "Proper Resolution For Insuring Adoptions to Distinctive Uniform." Over the next several years, some departments designed and publicized their own uniforms, which led to a need for centralizing Legion representation. In 1938, an official American Legion uniform was first offered in the Emblem Sales catalog: a blue serge military-style coat with matching trousers. The catalog advised posts to form a uniform committee to deal with local dealers.
In the freewheeling 1960s, standards relaxed. In fall 1963, the NEC approved an alternate uniform of a navy blazer (with Legion emblem) and gray trousers, which was offered in the Emblem Sales catalog in 1964. And the next year the "uniform" was cut down to just the cap.
The Emil A. Blackmore Museum at National Headquarters has on display full uniforms from the post-1938 era for a man, woman and young boy. And should you wish to get more fitted out for Legion events than your cap, both uniform versions are still available through Emblem Sales.