Based on first-principles calculations, we investigate the structural, mechanical and electronic properties of monolayer tin dioxides and dichalcogenides SnX₂ (Xdouble bondO, S, Se, Te) under uniaxial and biaxial strains. Our results show that monolayer 1T-SnX₂ is energetically more stable than 2H-SnX₂, while the stiffness of 2H-SnX₂ is much higher. The unstrained SnX₂ is thermodynamically and dynamically more stable than the strained ones. We also show that when external strain is applied, the band gaps of both 2H- and 1T-SnO₂ decrease linearly with increasing strain, which is contributed by the strain induced orbital redistribution of the decomposition of the p orbital of X atom and s, p orbitals of Sn atom. Notably, the slope of the band gap variation of the biaxial strained 1T-SnO₂ reaches up to -0.16 eV/1%. In our calculations, strain can result in a semiconductor–metal transition of 2H-SnX₂, while it only affects the band gap of 1T-SnX₂.