This dissertation critically evaluates the application of elemental and isotopic ratios as paleoredox tracers in early Earth’s oceans and on early Mars’ surface. It begins by questioning a common assumption in the field regarding the constancy of ocean mixing timescale throughout Earth’s history and demonstrates with an Earth system model that the ocean mixing timescale has indeed only fluctuated between a few hundred and a couple of thousand years since the Archean. This foundational understanding informs subsequent analyses of neodymium (Nd) and uranium (U) isotopes as proxies for early oceanic conditions. Nd isotope modelling suggests that the heterogeneous Nd isotopic compositions recorded in Archean sedimentary archives primarily reflect continental and sedimentary influences, with a global Nd residence time on the order of the ocean mixing timescale in the Archean; a U mass balance model indicates a significantly reduced global U residence time in the anoxic Precambrian oceans compared to today, highlighting the importance of understanding the biogeochemical cycles of (redox sensitive) proxies in early oceans prior to the interpretation of sedimentary records.Extending beyond terrestrial environments, this dissertation explores the potential of iron-manganese and iron isotopic fractionations as tracers of redox conditions during aqueous alteration of mafic minerals on early Mars. Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements of Martian clay analogs confirm a strong redox control on equilibrium iron isotopic fractionation during clay formation, and an iron oxidation kinetics-based alteration model exhibits good model-data correlation with paleosol records >1.85 Ga, promising a novel framework for inferring pedogenic redox conditions on Mars from future sample returns. Together, these findings not only underscore the intricate understanding necessary for interpreting sedimentary archives with paleoredox tracers, but also bridge geochemical insights between early Earth and Mars. By questioning long-held assumptions and introducing novel methodologies, this dissertation significantly enhances our geochemical toolkit for investigating the redox evolution on the surface of planetary bodies.