The Southern Canadian Cordillera has an average elevation of ~1.1 km and is characterized by widespread sporadic basaltic volcanism. Geophysical studies show that this area has a hot, thin lithosphere (~65 km thick). The origin of the thin lithosphere is unclear. One hypothesis is that it was inherited from Cordillera accretion at >100 Ma. An alternate idea is that it was created through gravitational thinning in the Eocene (~55 Ma). We use 2D numerical models to investigate these hypotheses. Model Set A explores a long-lived thin lithosphere. Models show that small-scale mantle convection is needed to maintain a 65 km lithosphere for >100 Ma; this requires a hydrated, low viscosity mantle (<1e20 Pa s). Model Set B examines thinning via delamination, using an initial lithosphere thickness of 120 km, including a 35 km crust. Delamination of the entire mantle lithosphere causes rapid crustal heating and surface uplift of ~1 km. The shallow mantle then starts to cool, forming new mantle lithosphere that thickens over time. There is only a short period 15-25 Myr after delamination where the model structure matches the observations, and therefore a Eocene delamination event alone cannot explain the thin lithosphere. We conclude that regardless of the origin of the thin lithosphere, at present-day, the Cordillera mantle is undergoing small-scale convection, and this provides the heat needed to sustain the high topography and basaltic magmatism. The two hypotheses may be discriminated by using observations of mantle lithosphere age and temporal variations in topography and magmatism.