Chiral perturbation theory is utilized to construct the renormalized magnetic masses and decay constants of the meson octet at next-to-leading order. While the neutral pion mass decreases identically to that in two-flavor chiral perturbation theory, the neutral kaon mass remains unaltered by the magnetic field. The renormalized magnetic masses for the charged mesons change identically. Meson decay constants in the axial and vector channels are constructed. Each decay constant increases monotonically in the magnetic background. Low-energy theorems – Gell-Mann-Oakes-Renner relations for the neutral mesons and their generalization for the charged mesons through the pseudoscalar coupling – are constructed and provide non-trivial crosschecks.