Catalyst-free InGaAs nanowires grown by selective area epitaxy are promising building blocks for future optoelectronic devices in the infrared spectral region. Despite progress, the role of pattern geometry and growth parameters on the composition, microstructure, and optical properties of InGaAs nanowires is still unresolved. Here, we present an optimised growth parameter window to achieve highly uniform In1−x Gax As nanowire arrays on GaAs (111)B substrate over an extensive range of Ga concentrations, from 0.1 to 0.91, by selective-area metal-organic vapor-phase epitaxy. We observe that the Ga content always increases with decreasing In/(Ga+In) precursor ratio and group V flow rate and increasing growth temperature. The increase in Ga content is supported by a blue shift in the photoluminescence peak emission. The geometry of the nanowire arrays also plays an important role in the resulting composition. Notably, increasing the nanowire pitch size from 0.6 to 2 µm in a patterned array shifts the photoluminescence peak emission by up to 120 meV. Irrespective of these growth and geometry parameters, the Ga content determines the crystal structure, resulting in a predominantly wurtzite structure for x Ga ≤ 0.3 and a predominantly zinc blende phase for x Ga ≥ 0.65. These insights on the factors controlling the composition of InGaAs nanowires grown by a scalable catalyst-free approach provide directions for engineering nanowires as functional components of future optoelectronic devices.