The effect of the Ce1–xLaxOy (x = 0–1, 1.5 ≤ y ≤ 2.0) support composition on the physicochemical properties of supported Ni catalysts and their activity in autothermal methane reforming was studied. The textural and structural characteristics of Ce1–xLaxOy and Ni/Ce1–xLaxOy samples and the process of their reduction in an atmosphere of hydrogen were examined using a set of techniques (low-temperature nitrogen adsorption, X-ray diffraction analysis, transmission electron microscopy, and thermal analysis). It was established that the Ce1–xLaxOy supports (x = 0–0.9) are mesoporous materials containing a fluorite-like solid solution based on cerium dioxide, in which the unit cell parameter increases and the average crystallite size decreases with the mole fraction of La. It was shown that the average size and composition of Ni-containing particles in the Ni/Ce1–xLaxOy catalysts depends on the composition of the support: at x = 0–0.8, a phase of NiO was formed, whereas a phase of LaNiO3 was formed at x = 0.9–1. The dispersity of the active constituent and its stability to agglomeration increased as the mole fraction of La in the Ce1–xLaxOy support was increased from 0 to 0.8, whereas the reduction of Ni-containing oxide particles shifted to the higher temperature region. The Ni/Ce1–xLaxOy catalysts provided high methane conversion (96–100%) and the yield of H2 (35–55%). The yield of hydrogen increased with decreasing the mole fraction of La in the Ce1–xLaxOy support composition; this can be caused by a decrease in the fraction of difficult-to-reduce Nin+ cations due to the weakening of metal–support interactions.