The current study investigated the relationship between underperformance, objective test performance, and subjective complaints in patients with persistent symptoms following mild traumatic brain injury (mTBI). The participants (N = 17) underwent a neuropsychological assessment. In the assessment, working memory was measured using the Number-Letter Sequence Test. Subjective complaints were evaluated with the Cognitive Failures Questionnaire (CFQ). Underperformance was assessed with the Amsterdam Short-Term Memory Test (AKTG), which is a performance validity test. Results indicated that five participants underperformed during the neuropsychological assessment. No relationship was found between objective test performance and subjective complaints, and no significant differences were found between the normal performance and underperformance groups regarding objective test scores or the number of subjective complaints. Future research with a larger sample and a broader range of cognitive tests is needed to further explore these findings and better understand the mechanisms underlying underperformance in mTBI patients.