Cancer-related cognitive decline refers to a deterioration in cognitive function affecting adults with cancer at any stage of their cancer journey. Older adults are at increased risk of cognitive decline. As the indications for immune checkpoint inhibitors expand in the treatment of cancer, understanding the potential complicating cognitive issues experienced by those receiving this therapy will be important. The aim of this scoping review is to identify the literature regarding immune checkpoint inhibitors and subjective/objective decline, to identify evidence in older adults, differences between older and younger adults, and outline areas for further research. Four large electronic databases were searched. Records were screened using standardised methodology. Ten studies were identified that met the inclusion criteria for review. Six studies objectively evaluated cognitive function in adults receiving ICI treatment; eight studies performed subjective cognitive assessments. There were differences identified in the cognitive assessment tools used and the methodology between studies. Few studies reported on age-dependent findings. The results of this scoping review highlight the need for further research in this area using standardised methodology and testing, with a particular focus on the cognitive outcomes of older adults who may be at increased risk of developing cognitive decline while on treatment.