We study momentum relaxation due to dilute, weak impurities in a strongly coupled CFT, a truncation of the M2 brane theory. Using the AdS/CFT correspondence, we compute the relaxation time scale as a function of the background magnetic field B and charge density {rho}. The theory admits two different types of impurities. We find that for magnetic impurities, momentum relaxation due to the impurity is suppressed by a background B or {rho}. For electric impurities, due to an underlying instability in the theory toward an ordered phase, the inverse relaxation time scale increases dramatically near {radical}(B{sup 2}+{rho}{sup 2}/{sigma}{sub 0}{sup 2}){approx}21T{sup 2}. We compute the Nernst response for the impure theory, and comment on similarities with recent measurements in superconductors.