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Abstract The composition of Earth's mantle, continental crust, and oceanic crust continuously evolve in response to the dynamic forces of plate tectonics and mantle convection. The classical view of terrestrial geochemistry, where mid‐ocean ridges sample mantle previously depleted by continental crust extraction, broadly explains the composition of the oceanic and continental crust but is potentially inconsistent with observed slab subduction to the lower mantle and oceanic crust accumulation in the deep mantle. We develop a box model to explore the key processes controlling crust‐mantle chemical evolution. The model mimics behaviors observed in thermochemical convection simulations including subducted oceanic crust separating and accumulating in the deep mantle. We demonstrate that oceanic crust accumulation strongly depletes the mantle independently of continental crust extraction. Slab stalling depths and continental crust recycling rates also affect the extent and location of mantle depletion. We constrain model regimes that reproduce oceanic and continental crust compositions using Markov chain Monte Carlo sampling. Some regimes deplete the lower mantle more than the upper mantle, contradicting the assumption of a more enriched lower mantle. All regimes require oceanic crust accumulation in the mantle. Though a small fraction of the mantle mass, accumulated oceanic crust can sequester trace element budgets exceeding the continental crust, depleting the mantle more than continental crust extraction alone. Oceanic crust accumulation may therefore be as important as continental crust extraction in depleting the mantle, contradicting the paradigmatic complementarity of depleted mantle and continental crust. Instead, depleted mantle is complementary to continental crust plus sequestered oceanic crust.