Halide double perovskite semiconductors such as Cs2AgBiBr6 are widely investigated as a more stable, less toxic alternative to lead–halide perovskites in photoconversion applications including photovoltaics and photoredox catalysis. However, the relatively large (~2.1 eV) and indirect bandgap of Cs2AgBiBr6 limits efficient sunlight absorption. Similar to lead–halide perovskites, the bandgap of double perovskites can be manipulated through (partial) substitution of metals or halides with similarly charged ions. However, commonly used solvent-based synthesis routes often lead to the formation of domains or side phases, rather than solid solutions with controlled properties. This results in an inhomogeneous electronic landscape which is detrimental for photoconversion applications. Here, we show that mechanochemical synthesis methods, such as ball milling, are a valid route to synthesize phase-pure double perovskites. With the use of synchrotron radiation we followed the formation mechanisms during mechanochemical synthesis of Cs2Ag[BiM]Br6 (with M = Sb, In, or Fe or X = Cl, Br, or I), and identified new intermediate phases, providing insights into the reaction kinetics. We find that mechanochemical synthesis is a successful approach to make compounds that have not been reported via solution-based synthesis routes, such as Cs2AgBi0.5In0.5Br6, and Cs2AgBi1-xFexBr6. Where substitution with In3+ increases the band gap energy, it is lowered when replacing Bi3+ with Fe3+ or Br− with I−. Hence, the optical bandgap of Cs2AgBiBr6 can be tuned over the entire visible spectrum when partly substituting Bi3+ or Br−. For instance, we find that controlled replacement of Bi3+ with Fe3+ via mechanochemical synthesis results in a remarkable tunability of the absorption onset between 2.1 to ~1 eV. In addition, with synchrotron-based high pressure X-ray diffraction (XRD), we find that the softness of these materials varies with its chemical composition. Our first-principles density functional theory (DFT) calculations demonstrate that this bandgap reduction originates from a lowering of the conduction band minimum upon introduction of Fe3+, while the valence band remains constant. Additionally, our DFT calculations suggest that the bandgap becomes direct when 50% of Bi3+ is replaced with Fe3+. Finally, we find that the tunability of the conduction band minimum is reflected in the photoredox activity of these semiconductors. The improved understanding of the reaction mechanism of alloyed-AgBi double perovskites might help to overcome the current challenges faced with solution processing methods. Hence, opening up new avenues for enhancing the visible light absorption of double perovskite semiconductors and for harnessing their full potential in sustainable energy applications.