Here’s an early (pre-Spencer) statement of the law of equal freedom and/or non-aggression principle:
[N]o movement can be permanently successful among progressive minds which stops short of a full and complete recognition of the entire liberty of the individual, so long as the action coming from such liberty trespasses upon neither the person or [sic] property of another.
Particularly interesting is its source: an 1837 publication of the First International; see details here. (The article is about Josiah Warren, but the quotation is not from Warren; in fact Warren wanted to make it less libertarian by including a right to one’s “reputation,” i.e., to the contents of other people’s minds.)
I am an idiot. Of course the First International didn’t exist in 1837. And so of course this quotation is from 1873, and so of course it’s not pre-Spencer.