We present a generalized cation anion codoping methodology for the synthesis of monodisperse, doped metal-oxide nanocrystals (NCs) that exhibit near-infrared localized surface plasmon resonance (LSPR) with the highest reported quality factors. We demonstrate that, in addition to the use of common cation dopants, the incorporation of fluorine into the lattice as an anion dopant can further increase the free-carrier concentration within individual NCs; this supports the cooperative effects of mixed cation anion doping in shifting the LSPR to higher energies. As a result, this method allows the LSPR of doped metal-oxide NCs to become tunable across a significantly broader wavelength range (1.5-3.3 pm), circumventing the prior limitations on the highest possible LSPR energies associated with single-element doping for a given oxide host. The strategy of cation anion codoping can offer new possibilities for the chemical design of doped semiconductor and metal-oxide NCs with tailored LSPR characteristics.