The longest alliance still in force is the Anglo-Portuguese alliance which was signed in 1386!

https://en.wikipedia.org/wiki/Anglo-Portuguese_Alliance

Good one. Not quite the same thing, but Saint Catherine's Monastery in the Sinai Peninsula claims to hold a covenant specifically guaranteeing their protection, signed by and possibly given to them in person by Muhammad: https://en.wikipedia.org/wiki/Ashtiname_of_Muhammad That would make it over 1400 years old. The authenticity is disputed, both whether it's original and whether it named and was given to them specifically, though AFAIU it's well known that Muhammad gave out that type of thing to various groups, for example Yemeni Christians[1]. Nonetheless, it seems to have been honored, albeit sometimes half heartedly, for most of the time since. The ironic thing is the greatest threat to its continuance is the nominally secular Egyptian state, which in its secularity feels less obliged, notwithstanding the motivation to infringe upon the monastery's autonomy principally arises from the Islamic culture and pressure groups.

[1] In the Yemeni case seemingly to forestall the Aksumite/Ethiopian Empire coming to their defense, which they had done pre-Islam when, IIRC, the Christians were slaughtered by the Yemeni Jewish kingdom.