We analyze spatially resolved and co-added SDSS-IV MaNGA spectra with signal-to-noise ratio �1/4100 from 2200 passive central galaxies (z ∼0.05) to understand how central galaxy assembly depends on stellar mass (M ∗) and halo mass (M h ). We control for systematic errors in M h by employing a new group catalog from Tinker and the widely used Yang et al. catalog. At fixed M ∗, the strengths of several stellar absorption features vary systematically with M h . Completely model-free, this is one of the first indications that the stellar populations of centrals with identical M ∗ are affected by the properties of their host halos. To interpret these variations, we applied full spectral fitting with the code alf. At fixed M ∗, centrals in more massive halos are older, show lower [Fe/H], and have higher [Mg/Fe] with 3.5σ confidence. We conclude that halos not only dictate how much M ∗ galaxies assemble but also modulate their chemical enrichment histories. Turning to our analysis at fixed M h , high-M ∗ centrals are older, show lower [Fe/H], and have higher [Mg/Fe] for M h > 1012 h -1 M ⊙ with confidence >4σ. While massive passive galaxies are thought to form early and rapidly, our results are among the first to distinguish these trends at fixed M h . They suggest that high-M ∗ centrals experienced unique early formation histories, either through enhanced collapse and gas fueling or because their halos were early forming and highly concentrated, a possible signal of galaxy assembly bias. © 2022. The Author(s). Published by the American Astronomical Society.