Direct functionalization of C(sp3)–H bonds allows a fast, synthetic access to valuable products, starting from simple petrochemicals. Chemical transformation of inactivated methylene groups, however, remains challenging for organic synthesis. Here, we present a general electrochemical method for the oxidation of C(sp3)–H and C(sp2)–H bonds, in which cyclic alkanes and (cyclic) olefins are converted into cycloaliphatic ketones as well as aliphatic (di)carboxylic acids. This resource-friendly method is based on nitrate salts in a dual role as anodic mediator and supporting electrolyte, which can be fully recovered and recycled. Reduction of molecular oxygen as cathodic counter reaction leads to an efficient convergent use of both electrode reactions. By avoiding the use of transition metals and chemical oxidisers, this protocol represents a sustainable oxo-functionalization method, leading to a valuable contribution for the sustainable conversion of petrochemical feedstocks into synthetically usable fine chemicals and commodities.