The ego-depletion effect has been interpreted as a temporary failure of self-control exertion after a first effortful task (Baumeister et al., 2007). Although the existence of this effect was previously challenged (Vohs et al., 2021), it was successfully replicated in a recent study applying an adequate experimental protocol (Mangin et al., 2021). In the present experiment, we applied a similar protocol while assessing the electrophysiological changes in effort engagement through mid-frontal theta and cardiac reactivity (Cavanagh & Frank, 2014; Mukherjee et al., 2011). A total of 32 participants in a between-subjects design performed the following task sequence: handgrip task at 13% of maximal voluntary contraction, a 30-min mental task (depleting or control task) and the handgrip task again (dependent task). The experimental group (N = 16) performed a modified Stroop task, while the control group (N = 16) watched an emotionally neutral documentary. EEG and ECG were recorded on a continuous basis. We assumed to observe an increase in the electrophysiological correlates of mental effort during the depleting task and a performance decrement in the subsequent dependent task. For the depleting condition, the behavioral results showed worse performance in the subsequent dependent task and the EEG results showed higher mid-frontal theta power. Heart-rate variability results showed an increase in parasympathetic activity over time during the depleting task. Therefore, the ego-depletion effect was once again successfully replicated. The depleting task required more mental effort than the control task, but a slight decrease in effort engagement over time was also observed.