Who said “Less is more”?
Mies van der Rohe is the common answer, and he certainly deserves full credit for making it a common phrase.
I seem to remember though running across it in a poem by Robert Browning (or maybe Robert Burns) once. I’ll see if I can scare up a citation.
It wasn’t Yogi Berra?
Well, it’s not the Harvard Review or anything, but here’s one site that agrees with me. 