Oxygen atom transfer by high-valent enzymatic intermediates remains an enigma in chemical catalysis. In particular, manganese is an important first-row metal involved in key biochemical processes, including the biosynthesis of molecular oxygen (through the photosystem II complex) and biodegradation of toxic superoxide to hydrogen peroxide by superoxide dismutase. Biomimetic models of these biological systems have been developed to gain understanding on the structure and properties of short-lived intermediates but also with the aim to create environmentally benign oxidants. In this work, we report a combined spectroscopy, kinetics and computational study on aldehyde deformylation by two side-on manganese(III)-peroxo complexes with bispidine ligands. Both manganese(III)-peroxo complexes are characterized by UV-vis and mass spectrometry techniques, and their reactivity patterns with aldehydes was investigated. We find a novel mechanism for the reaction that is initiated by a hydrogen atom abstraction reaction, which enables a keto-enol tautomerization in the substrate. This is an essential step in the mechanism that makes an electrophilic attack on the olefin bond possible as the attack on the aldehyde carbonyl is too high in energy. Kinetics studies determine a large kinetic isotope effect for the replacement of the transferring hydrogen atom by deuterium, while replacing the transferring hydrogen atom by a methyl group makes the substrate inactive and hence confirm the hypothesized mechanism. Our new mechanism is confirmed with density functional theory modeling on the full mechanism and rationalized through valence bond and thermochemical cycles. Our unprecedented new mechanism may have relevance to biological and biomimetic chemistry processes in general and gives insight into the reactivity patterns of metal-peroxo and metal-hydroperoxo intermediates in general.