Monomer sequence is shown to directly control interfacial and in-domain mixing in lamellar polystyrene-b-poly-peptoid diblock copolymers, where the polypeptoid block is composed of precise sequences of highly segregating (polar) and compatibilizing (nonpolar) repeat units. With monomer-by-monomer sequence control, the effects of blockiness, comonomer distribution, and taper direction on the interfacial width are measured with small-angle X-ray scattering and further understood through simulations with self-consistent field theory. When compatibilizing groups are distributed along the polypeptoid chain, the presence of neighboring polar groups suppresses interfacial mixing and causes the interfacial width to narrow, especially for the blocky sequence. When compatibilizing groups are tapered from the block junction, they are strongly localized at the domain interface and both interfacial mixing and interfacial width are increased. Consequently, tapered sequences produce more pure domain centers, while the distributed materials have compatibilizing groups located throughout the polypeptoid domain, encouraging more polystyrene to mix in. An analogous poly(n-butyl acrylate) system was synthesized to probe the effects of a different interaction parameter (chi). Similar trends with sequence were found but with smaller magnitudes due to the higher chi. This study shows that monomer sequence directly affects segmental mixing both at and away from the interface, with consequences for interfacial width and domain spacing. Along with the sequence-driven nonideal chain conformations shown recently, these combined sequence-specific effects determine the resulting geometry and thermal stability of the self-assembled lamellae, suggesting that comonomer sequence can be used to tailor self-assembling materials without changing the composition or chemistry.