Mantle convection and, by association, plate tectonics is driven by the transport of heat from a planetary interior. This heat comes from the internal energy of the mantle and from heat flowing into the base of the mantle from the core. Past investigations of such mixed mode heating have revealed unusual behavior that confounds our intuition based on end‐member cases. In particular, increased internal heating leads to a decrease in convective velocity. We investigate this behavior using a suite of numerical experiments and develop a scaling for velocity in the mixed heating case. We identify a planform transition, as internal heating increases, from sheet‐like to plume‐like downwellings that impacts heat flux and convective velocities. More significantly, we demonstrate that increased internal heating leads not only to a decrease in internal velocities but also a decrease in the velocity of the upper thermal boundary layer (a model analog of the Earth's lithosphere). This behavior is connected to boundary layer interactions and is independent of any particular rheological assumptions. In cases with a temperature‐dependent viscosity and weak plate margin analogs, increased internal heating does not cause an absolute decrease in surface velocity but does cause a decrease relative to purely bottom or internally heated cases as well as a transition to rigid‐lid behavior at high heating rates. The differences between a mixed system and end‐member cases have implications for understanding the connection between plate tectonics and mantle convection and for planetary thermal history modeling. Key Points In mixed heated thermal convection, velocities can decrease with increased internal heating Scaling theory and numerical models show that velocity trends reflect changing levels of boundary layer interaction The energtics of mixed heating mantle convection differs from bottom and internally heated end‐members