The effectiveness and mechanism of dimethyldithiocarbamate (DDTC) for recovering low-grade cobalt and nickel from the lixivium of a manganese-rich slag have been investigated via a computational and experimental study. First principles calculations using a solvation model showed that DDTC can efficiently bind Fe(III), Co(II), Ni(II), and Cu(II) via the carbon disulfide functional group, indicating that Fe(III) and Cu(II) should be removed before addition of DDTC. Experimentally, 80.2% Co and 78.0% Ni were extracted from the slag containing low-grade 0.09% Co and 0.073% Ni under optimal conditions. Then, 99.92% of iron was removed from the lixivium via hydroxide precipitation. Finally, after the treatment with DDTC, excellent recoveries of 97.98% and 98.08% were obtained for Co and Ni, respectively. Further, high-resolution X-ray photoelectron spectroscopy results showed that the added DDTC can collect not only the free metal ions but also the nano-sized sulfide mineral particles. Thus, DDTC shows potential for the recycling of low-grade Co and Ni resources from solution.