neieio:
First, welcome to Progarchives. We hope you are enjoyin yourself, and we look forward to your participation and contributions both to the reviews and the forums.
This thread (earliest progressive song) has, of course, been done many times. You may want to check out some of the older threads under "Main Discussions," as there may still be one of them around.
That said, the position I take is that The Beatles wrote the first "proto-prog" songs, including "Tomorrow Never Knows" (1966), and "I Am the Walrus" and "Strawberry Fields Forever" (1967). However, Frank Zappa may have been earlier, with "proggish" stuff as early as 1966, or even 1965. Then there are those who consider "Telstar" - an instrumental single (1962!) - as the first proto-prog song.
Still, you are not the first to suggest MacArthur Park, and it does have an illustrious past and place in music history.
Let's see what others have to say...
Again, hope you enjoy yourself here. It can be great fun!
Peace.