  We conduct a comprehensive study on the sub-GeV millicharge dark matter
produced through the freeze-in mechanism. We discuss in general the mixing
mechanism, encompassing both kinetic mixing and mass mixing, between the
$U(1)_X$ hidden sector and the standard model, which can generate millicharge
carried by the dark fermions from the hidden sector. We discuss in depth how
such millicharge is generated, and clarify several misunderstandings regarding
this subject in the literature. Without employing an effective field theory
approach, where the photon field directly mixed with the additional $U(1)$, we
analyze a general renormalizable model and investigate the complete evolution
of the hidden sector particles. Due to the substantial self-interactions among
hidden sector particles, the evolution of the hidden sector temperature plays a
crucial role, which is addressed concurrently with the number densities of
hidden sector particles by solving a set of coupled Boltzmann equations. We
thoroughly examine eight benchmark models from six distinct cases. Some of our
key findings from the analysis of these benchmark models may be generalizable
and applicable to broader freeze-in scenarios. We also explore the possibility
that the $\mathcal{O}$(keV) $U(1)_X$ dark photon is a viable dark matter
candidate, even though it can contribute at most $\sim 5\%$ to the total
observed dark matter relic density.
