We present the first synthesis of Mn I lines in realistic multi-component magnetized quiet Sun atmospheres, including variation of magnetic field strength, magnetic field direction, plasma velocity, and coupling between magnetic field and thermodynamics. All synthesized Mn I lines weaken for strong magnetic fields. A detailed analysis of the Mn I λ5538 line shows that the Weak Field Approximation (WFA) breaks down at 400~G, and that kG magnetic fields produce Stokes V profiles with amplitudes up to two orders of magnitude smaller than those predicted by the WFA. Consequently, the polarization emerging from an atmosphere in which weak (hG) and strong (kG) fields coexist is biased towards the hG field polarization. When including velocity gradients, profiles showing important asymmetries are produced, and these profiles cannot be obtained using single-component magnetized model atmospheres. We synthesize Mn I lines presenting very different hyperfine structure (HFS) patterns, and all of them reproduce the observed Stokes I profiles in a really satisfactory way.