The structurally related compounds NiI₂ and CoI₂ are multiferroic van der Waals materials in which helimagnetic orders exist simultaneously with electric polarization. Here, we report on the evolution of the crystal structure and of the magnetic properties across solid solution Co_1–xNi_xI₂. We have successfully grown crystals of the whole range of the solid solution, i.e., x = 0–1, by employing the self-selecting vapor growth (SSVG) technique and by carefully tuning the synthesis conditions according to the chemical composition. Our structural investigations show that the crystal symmetry changes from P3̅m1 to R3̅m when Ni substitutes for Co beyond x = 0.2. Both the lattice parameters and magnetic properties evolve continuously and smoothly from one end member to the other, showing that they can be finely tuned by the chemical composition. We also observe that the degree of Ni substitution in the solid solution affects the metamagnetic transition typical for CoI₂ at high magnetic fields. In particular, we find the existence of a metamagnetic transition similar to that for CoI₂ in the NiI₂ structure. Based on magnetic measurements, we construct the phase diagram of the Co_1–xNi_xI₂ system. Controlling the magnetic properties by the chemical composition may open new pathways for the fabrication of electronic devices made of two-dimensional (2D) flakes of multiferroic van der Waals materials.