Although it does not use the term "net neutrality" (which I had never seen before about 5 years ago), the 1956 "Consent Decree" (a kind of court ruling), which resolved the second anti-trust suit against AT&T, imposed a version of net neutrality on the U.S. telephone network, and I have seen at least one writer credit that "Consent Decree" with enabling the growth of the internet beyond the purely government-operated stage (because the growth of the internet at that stage came from large organizations' leasing "dedicated copper" from AT&T Long Lines, which wanted to but was was unable to turn down requests for such leases because of the Consent Decree).

This version of net neutrality was restricted to large corporations (IBM was a very vocal advocate of this version of net neutrality) and the judicial system, and had almost nothing to do with popular opinion or electoral politics, but it was the law in the U.S.