We perform joint inversions for mantle viscosity of geophysical observables associated with both mantle convection and glacial isostatic adjustment (GIA). Our data include coefficients of the non-hydrostatic geoid (up to degree 8) and decay times associated with post-glacial relative sea level (RSL) variations in Hudson Bay and Fennoscandia. We find that both data sets may be reconciled using a single profile of mantle viscosity which is characterized by a significant increase, with depth, across this region. This result weakens previous arguments that the mantle rheology has significant transient effects. The viscosity profiles we obtain are found to reconcile the observed free air gravity anomaly over Hudson Bay. ¿ American Geophysical Union 1996