We consider self-annihilation of dark matter, $\chi$, into metastable mediators, $Y$, and their subsequent decay into photons inside white dwarfs. We focus on reactions of the type $\chi \overline{\chi }\to YY$, where mediators, besides having a finite decay lifetime at rest ${\tau }_{\mathrm{rest}}\lesssim 1s$, may suffer energy loss in the medium before they decay into photons, $Y\to \gamma \gamma$. We obtain attenuated gamma-ray luminosities arising from the combination of both effects. Using complementary sets of astrophysical measurements from cold white dwarfs in the M4 globular cluster as well as direct and indirect dark matter searches, we discuss further constraints on dark mediator lifetimes.