Purpose Patients with advanced cancer often experience cognitive dysfunction, which may influence decision making, self-perception, and existential well-being. However, there is little evidence regarding this issue. This study analysed associations between objective neuropsychological measures and patients' self-report of cognitive dysfunction interfering with everyday life, general well-being, and sense of existential value. Methods A mixed method study assessed 13 adult patients with advanced cancer with validated neuropsychological tests, which assessed sustained attention, psychomotor speed, memory/attention, mental flexibility, and a measure of global cognitive function. These were followed by semi-structured interviews focusing on subjective experiences of cognitive dysfunction. Agreement between subjective and objective measures were analysed by Cohen's Kappa (k). Thematic analysis explored associations with cognitive deficits. Results Poor cognitive performance on the neuropsychological tests was observed regarding sustained attention (n = 8), psychomotor speed (n = 1), memory/attention (n = 2), mental flexibility (n = 9), and global cognitive function (n = 3). Almost all patients (n = 12) had complaints of cognitive dysfunction. However, the agreement between the two assessments was weak (k