In this study, zinc-gallium oxynitrides with a Zn:Ga mole ratio of 1:1 [(GaN)0.5(ZnO)0.5] were synthesized from a Zn/Ga/CO3 layered double hydroxide (LDH) precursor. The microstructure and photoactivity of the (GaN)0.5(ZnO)0.5 particles were tuned by adjusting the nitridation conditions of the LDH. It is revealed that the quantity of the LDH, or, equivalently, the partial pressure of the water during nitridation, plays a pivotal role in the defect structure of the obtained oxynitrides. A reduction in the quantity of the LDH precursor can effectively suppress the formation of defects including Ga(Zn)-O bonding, bulk anion vacancies, and surface-deposited Ga/ON···VGa complexes, leading to a better charge-separation efficiency for the photogenerated electron-hole pairs in the oxynitride. Furthermore, a suitable introduction of methane during nitridation would not only increase the crystallinity of the bulk materials but also enhance the density of the surface oxygen vacancy (VO), which would raise the charge-injection efficiency by working as an electron trap and a reaction site to form O2•-. O2•-, as well as photogenerated holes, have been proven to be the dominant active species for the photodegradation of phenol. 25CH4-ZnGaNO, with the lowest density of bulk defects and the highest density of surface VO, exhibited the best photoactivity under visible-light irradiation for the photodegradation of Rhodamine B and phenol. The obtained surface-VO-rich (GaN)0.5(ZnO)0.5 particles can be applied as a high-performance visible-light-driven photocatalyst in the photodegradation of organic pollutants.