<p>Capturing the active state of (electro)catalysts under operating conditions, namely <i>operando</i>, is the ultimate objective of (electro)catalyst characterization, enabling the unraveling of reaction mechanisms and advancing (electro)catalyst development. <i>Operando</i> insights advance our understanding of the correlations between electrochemical tests and device-level performances. However, <i>operando</i> characterization of electrocatalysts is challenging due to the complexity of electrochemical devices and instrumental limitations. As a result, the majority of electrocatalyst characterizations have been limited to half-cell in situ studies. Here, we present an <i>operando</i> X-ray absorption spectroscopic study of Mn spinel oxide electrocatalysts in an operating fuel cell employing a custom-designed cell. Our results reveal that in anion exchange membrane fuel cells, the Mn valence state, within spinel Mn₃O₄/C, increases to above 3+, adopting an octahedral coordination devoid of Jahn-Teller distortions. This structural change results in an AEMFC performance equivalent to that of Co_1.5Mn_1.5O₄/C, a composition that outperforms Mn₃O₄/C in rotating disk electrode tests. Our results underscore the importance of <i>operando</i> characterizations in elucidating the active state of electrocatalysts and understanding the correlation(s) between electrochemical tests and device performance.</p>