Multicomponent Mannich reactions through C−H bond activation are described. These transformations allowed for the straightforward generation of densely substituted benzylic and homo‐benzylic amines in good yields. The reaction involves a reaction between two transient species: an organometallic species, generated by transition‐metal‐catalyzed sp2 or sp3 C−H bond activation and an in situ generated imine. The use of an acetal as an aldehyde surrogate was found essential for the reaction to proceed. The process could be successfully applied to RhIII‐catalyzed sp2 C−H bond functionalization and extended to CuII‐catalyzed sp3 C−H bond functionalization.