We report composition dependent structure evolution from SrTiO₃ to SrFe_0.5Ta_0.5O₃ by powder X-ray and neutron diffraction studies of SrTi_1−2xFe_xTaxO₃ (0.00 ≤ × ≤ 0.50) compositions. Structural studies reveal cubic (Pm3m) perovskite-type structure of the parent SrTiO₃ for x up to 0.075 and cation disordered orthorhombic (Pbnm) perovskite-type structure for x ≥ 0.33. A biphasic region consisting of a mixture of cubic and orthorhombic structures is found in the range for 0.10 ≤ × ≤ 0.25. Dielectric studies reveal transformation from a normal dielectric to relaxor like properties with increasing Fe³⁺ and Ta⁵⁺ concentration. Dielectric response is maximum at x = 0.33 in the series. The results establish a protocol for designing new lead-free relaxor materials based on the co-substitution of Fe³⁺ and Ta⁵⁺ for Ti⁴⁺ in SrTiO₃. A complex interplay of strain effects arising from distribution of cations at the octahedral sites of the perovskite structure controls the dielectric properties.