We present results of a comprehensive experimental study of the compound SmCrTiO5, belonging to the RM2O5 family, where M is a mixture of two transition metals, Cr3+ and Ti4+. The studies were carried out using temperature-dependent synchrotron x-ray- and neutron-powder diffraction, and measuring the temperature- and field dependences of magnetization. No structural transition is observed between 2 and 450 K. The crystal structure of SmCrTiO5 is described in the orthorhombic space group Pbam. It is built from MO5 square pyramids and MO6 octahedra. The Sm3+ cations occupy a polyhedron of coordination 8, which shares edges with the two other polyhedra. Cr3+ and Ti4+ are found to differentially occupy each of the two transition-metal coordination sites, forming two substructures. Magnetic d3Cr3+ ions occupy the octahedra which share edges to form chains, while nonmagnetic d0 Ti4+ ions occupy the pyramids. Magnetic susceptibility curves present a broad signal below 195 K with a maximum at about 100 K, characteristic of 1D antiferromagnetism. Nevertheless, long-range 3D magnetic order appears only below 12 K as evidenced by neutron-powder diffraction. The refined magnetic structure is in addition supported by electronic structure calculations. From these data, a model of spin ordering in the ground state is proposed. Since the structural features are characteristic of alternating Heisenberg antiferromagnetic chains, the magnetic properties of SmCrTiO5 are compared with its SmCrGeO5 analog for which a spin gap was evidenced.