Defect engineering of La1-xSrxMnO3 (LSMO)-a strongly correlated oxide displaying half metallicity and ferromagnetism above room temperature-has been the focus of a long-standing quest aimed at the exploitation of this material as a functional building block for memory storage and spintronic applications. Here, we discuss the correlation between structural defects and magnetism in La0.74Sr0.26MnO3/SrTiO3 (LSMO/STO) epitaxial heterostructures as a function of growth temperature and post-deposition annealing. Upon increasing the growth temperature from 500 to 700 °C at a fixed oxygen partial pressure of 0.007 mbar, the sputter-deposited epitaxial LSMO films experience a progressive increase in Curie temperature Tc from 110 to 270 K and saturation magnetization Ms from 1.4 to 3.3 μB/u.c. owing to a reduction in oxygen deficiencies. Concurrently, however, growth temperatures above 600 °C trigger the formation of off-stoichiometric, dendritic-like SrMoOx islands at the film/substrate interface as a possible aftermath of temperature-driven diffusion of impurities from the STO substrate. Notably, although the interfacial spurious islands cause an increase in sample surface roughness, the heterostructure still preserves high-quality epitaxy. In general, the best compromise in terms of both structural and magnetic properties, comprising high-quality epitaxy, atomically flat surface, and robust ferromagnetism above room temperature, is obtained for LSMO films grown at a relatively low temperature of about 500-540 °C followed by a post-deposition annealing treatment at 900 °C for 1 h in air. Our study compares effective routes based on temperature-controlled defect engineering to finely tailor the complex interplay between microstructure and magnetism in LSMO thin films.