Magnesium silicides are favorable thermoelectric materials considering resource abundance and cost. Chromium (Cr) doping in magnesium silicides has not yet been explored. Using first-principles calculations, we have studied the stability of Mg2Si with chromium (1.85, 3.7, 5.55, and 6.25% Cr) and tin (12.5 and 50% Sn). Three Mg2Si compounds doped with Sn, (Sn + Bi), and (Sn + Bi + Cr) are used to explain doping effects on thermoelectric performance. Notably, Cr behaves nonmagnetically for ≤2%Cr, after which ferromagnetic ordering is favored (≤12.96% Cr), despite its elemental antiferromagnetic state. With alloying of Sn (70.4%), Mg2Si remains an indirect-band-gap semiconductor, but adding small amounts of Bi (3.7%) increases the carrier concentration such that electrons occupy conduction bands, making it a degenerate semiconductor. Mg2Si0.296Sn0.666Bi0.037 is found to give the highest thermoelectric figure of merit (ZT) and power factor (PF) at 700 K, i.e., 1.75 and 7.04 mWm−1K−2, respectively. Adding small %Cr decreases ZT and PF to 0.78 and 4.33 mWm−1K−2, respectively. Such a degradation in thermoelectric (TE) performance is attributed to two factors: (i) uniform doping acting as an electron acceptor, decreasing conduction, and (ii) the loss of low-lying conduction band degeneracy with doping, decreasing the Seebeck coefficients. A study of configurations of Cr doping suggests that Cr has a tendency to form clusters inside the lattice, which play a crucial role in tuning the magnetic and TE performance of doped Mg2Si compounds.