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The relative ease of tuning the properties of two-dimensional materials compared to their three-dimensional counterparts offers great potential to achieve previously inaccessible multifunctional devices. In this Dissertation, we use strain engineering as a non-destructive way to control the properties of two-dimensional materials, employing density functional theory and chemical vapor deposition. In the first part of the Dissertation, density functional theory is used to investigate the effect of biaxial strain on the structural, electronic, and magnetic properties of pristine and Janus Cr-trihalide monolayers. We find that the broken inversion symmetry of the Janus monolayers X3-Cr2-Y3 (X, Y = Cl, Br, and I) enhances their functionality by making the magnetic anisotropy tunable by strain and inducing an out-of-plane electric polarization. A very negative magnetic anisotropy energy of ̶ 3.77 meV per formula unit is realized in the Cl3-Cr2-I3 monolayer under ̶ 5% strain. In the second part of the Dissertation, we perform a comprehensive investigation of thermally strained monolayer MoS2, both theoretically and experimentally, to tune the sulfur vacancy density. Due to a dominant role of the intralayer electrostatic interaction, compressive (tensile) biaxial strain decreases (increases) the sulfur vacancy formation energy and, thus, increases (decreases) the probability of creating sulfur vacancies. This fundamental relationship opens a new venue for defect engineering of transition metal dichalcogenides.