Electron magnetic resonance (EMR) studies of iron-gallium borate, FexGa1−xBO3, single crystals have been carried out in the frequency range ca. 8–38 GHz in magnetizing fields up to 10 kOe and the temperature range of 4–310 K. With decreasing x in the range of 0.34≤x≤1, the EMR spectra show a gradual passage from a low-frequency antiferromagnetic resonance (AFMR) mode at x = 1 toward a coexistence of AFMR and cluster magnetic resonance arising, respectively, from completely and partially magnetically ordered crystal regions. Temperature and concentration dependences of magnetic characteristics of iron-gallium borates, namely, the Néel temperature, the Dzyaloshinskii–Moriya field, and the isotropic energy gap, have been determined by means of AFMR. In contrast to unmixed FeBO3, FexGa1−xBO3 crystals with 0.34≤x≤0.85 show anomalous nonmonotonic temperature dependences of the Dzyaloshinskii–Moriya field with a maximum well below the Néel temperature suggesting the occurrence of another magnetic transition in this temperature range.