Participants worked on an easy versus difficult arithmetic task with integrated happiness versus sadness primes, presented either suboptimally (briefly and masked) or optimally (long and visible). As predicted by the IAPE model (Gendolla in International Journal of Psychophysiology 86:123–135, 2012. doi:10.1016/j.ijpsycho.2012.05.003), the affect primes moderated the task difficulty effect on mental effort in the suboptimal-prime condition: cardiac pre-ejection period response was stronger in the happiness/difficult than in the sadness/difficult condition and tended to be stronger in the sadness-easy than in the happiness-easy condition. These effects were reversed in the optimal-prime-presentation condition, suggesting behavior correction due to controlled prime processing. Moreover, neither suboptimally nor optimally presented affect primes had prime-congruent effects on conscious mood assessed via self-report. The results demonstrate differential effects of implicitly versus explicitly processed affect cues on mental effort and suggest that they can do so without inducing emotional feelings.