Herein, we leveraged lithium salt doping of linear and cyclic block polymers (BPs) in thin film geometries to demonstrate how BP architecture influences self-assembled nanofeature sizes and interfacial widths in nanostructured thin films as a function of segregation strength. To mitigate potential issues with sample-to sample variability and time-consuming synthesis, we used a single linear BP specimen that was ring-closed to generate an analogous cyclic BP, and we mixed the cyclic and/or parent linear BP with a lithium salt to modulate effective segregation strength (chi N-eff). Based on X-ray reflectivity analyses, cyclic polystyrene-block-poly[oligo(ethylene glycol) methacrylate] BP thin film specimens had similar to 20% smaller domain spacings than their linear counterparts at all chi(eff)Ns and lower absolute sensitivities to changes in chi(eff) at identical molecular weights. We also report the first direct measurements of interfacial widths in cyclic BP assemblies, which quantitatively demonstrated that interfacial mixing in cyclic BPs was greater relative to linear BPs. Furthermore, the trends in domain characteristics with increasing salt concentration qualitatively agreed with results from molecular dynamics (MD) simulations with increasing chi, despite the fact that salt species were not explicitly included in the MD simulations. Our results underscore the utility of lithium salt doping to explore the BP phase behavior of synthetically challenging macromolecules and demonstrate key architecture/segregation strength relationships in cyclic BP thin films, which provides useful information to further evaluate cyclic BP suitability for nanoscale patterning and templating applications.