The effects of low carrier density and carrier compensation on mixed-state thermal transport are investigated beyond the quasiclassical approximation. It is shown that, contrary to the usual observations, the interplay of the two effects leads to an increase in the thermal conductivity immediately below the upper critical field H_c2. Our result can account for the anomalous behavior of the mixed-state thermal conductivity near H_c2 recently observed in URu_2Si_2.