The unique combination of outstanding optical quality and attractive spin properties opens new avenues for optical spin control in hybrid organic-inorganic perovskite semiconductors. Using the optically detected magnetic resonance technique, we study the spins of electrons and holes in mixed-cation MAₓFA₁₋ₓPbI₃ single crystals with x = 0.4 and 0.8. Multiple distinct spin subensembles with g-factors spanning from 2.9 to 3.6 for electrons and from 0.5 to 1.2 for holes are resolved, revealing diverse localization environments. We measure the longitudinal spin relaxation times, T₁, reaching 2 ms and remaining in the μs range even for weakly localized carriers at the cryogenic temperature of 1.6 K. The magnetic-field dependence of T₁ is dominated by the random nuclear (Overhauser) fields with strengths of ∼ 0.4-0.8 mT for electrons and ∼ 4-12 mT for holes, corresponding to μs-long correlation times of the hyperfine field determined by carrier hopping between shallow localization sites. The temperature dependence of T₁ reveals a weak localization potential of the charge carriers and shows a correlation between T₁ and the inhomogeneity of the spin ensemble. These results establish mixed-A-site perovskite single crystals as a promising solid-state platform with long-lived spin states for quantum information applications.