Traits related to dispositional differences in cognitive effort investment (CEI) like need for cognition (NFC) and self-control have been shown to be related to effort-based decision making. Based on previous findings, we assumed that CEI, as an integrative measure of the willingness and tendency to exert effortful control, would be related to behavioral and psychophysiological indices of effort investment in a typical cognitive control task under varying demand and payoff. Specifically, performance and midfrontal theta power (FMθ) in the electroencephalogram are sensitive to cognitively demanding tasks and were shown to exhibit different patterns for individuals with high vs. low NFC as one core aspect of CEI: performance decreased more strongly for individuals with low vs. high NFC, and high NFC individuals showed a demand-congruent increase in FMθ, pointing to a more efficient allocation of cognitive resources. In the present study (N ~ 145), we examined the relationship of CEI and behavioural measures, i.e., reaction time (RT), error rate (ER) and FMθ, during a flanker task with varying demand and payoff. Whereas the analysis of the behavioral data revealed significant effects for demand (RT, ER), demand x payoff (RT, ER) and payoff x CEI (RT), significant effects regarding FMθ emerged for demand and demand x CEI. Taken together, we mapped CEI onto objective markers of the willingness to exert cognitive effort. Our result may further our understanding of person x situation interactions with regard to effort investment in goal-directed behavior.