Sunlight-induced photoformation of silver nanoparticles (nAg), mediated by natural organic matter (NOM), is significantly affected by the concentration of Ag(I) and chloride. The initial photoformation rates of nAg in Suwannee River humic acid (SRHA) and Suwannee River natural organic matter (SRNOM) solutions were examined under simulated sunlight irradiation. A critical induction concentration (CIC) of Ag(I) (10 mg/L for SRHA and 5 mg/L for SRNOM, respectively) was observed, below which the nAg formation was minimal. The threshold is attributed to the interplay of reduction and oxidation reactions mediated by NOM, reflecting the need to achieve sufficiently fast growth of silver clusters to outcompete oxidative dissolution. The CIC can be reduced by scavenging oxidative radicals or be increased by promoting singlet oxygen and hydrogen peroxide generation. The presence of chloride effectively reduced the CIC by forming AgCl, which facilitates reduction reactions and provides deposition surfaces. SRNOM is more efficient in mediating photoformation of nAg than SRHA, owing to their differed phototransient generation. These results highlight prerequisites for the photoformation of nAg mediated by NOM, in which the photochemistry and solution chemistry are both important.