Metal halide perovskites have emerged as a new class of materials that are promising for semiconductor spintronics. Here we report a systematic investigation of coherent spin precession, spin dephasing and spin relaxation of electrons and holes in two hybrid organic-inorganic perovskites MA_0.3FA_0.7PbI₃ and MA_0.3FA_0.7Pb_0.5Sn_0.5I₃ using time-resolved Faraday rotation spectroscopy. With applied in-plane magnetic fields, we observe robust Larmor spin precession with spin dephasing times of hundreds of picoseconds. From these measurements, we determine the Lande g-factor, the inhomogeneous broadening of the g-factor, and spin dephasing time for both electrons and holes. Temperature-dependent measurements give further insight into the spin relaxation mechanisms and the strong influence of the lattice distortion on the g-factors. These results lay the foundation for further design and use of these new semiconductors for spin-based electronics.