Recent reports of superconductivity and magnetism in single crystals of [Li1-xFexOH]FeS show unexplained variations in both superconducting and magnetic properties. We investigate the effect of hydrothermal growth conditions on these properties and find that increasing the growth temperature systematically increases the superconducting transition temperature (T-c), sharpens the magnetic transition, and decreases the scattering rate (Gamma). The slow rate of T-c suppression with increasing Gamma indicates a conventional s-wave superconducting state according to the Abrikosov-Gorkov expression. Samples with higher scattering rate show broader magnetic transitions and a stronger temperature dependence in the magnetic susceptibility. These results identify disorder, due to interstitial iron impurities, as the unique internal parameter responsible for the unexplained variations in T-c and magnetic ordering. We demonstrate the optimal hydrothermal growth conditions to minimize disorder and maximize T-c in [Li1-xFexOH]FeS crystals.