Objective: The objective of this study is to identify factors determining the time to diagnosis for young-onset dementia (YOD), defined as dementia with symptom onset before age 65 years, by mapping the diagnostic pathways. Methods: Participants were recruited via healthcare professionals, community support organisations or were self-referred. Information was obtained by interviews with the person with YOD and their carer, and medical record reviews. Clinical dementia diagnoses were independently ratified by consensus review. Results: Participants included 88 people with YOD (mean age of onset = 55.4 years), due to Alzheimer's disease (AD) (53.4%, n = 47), frontotemporal dementia (FTD) (15.9%, n = 14) and other causes (30.7%, n = 27). Median time from symptom onset to first consultation was 2.3 years, to dementia diagnosis 3.2 years, to family awareness of dementia diagnosis 3.5 years and to final diagnosis of the type of dementia 4.7 years. Non-dementia diagnoses occurred in 48.9%, including depression (30.7%) and mild cognitive impairment (MCI) (17.0%). Participants with younger age of onset had significantly longer time to first consultation and family awareness of the dementia diagnosis. The time to dementia diagnosis was significantly longer when the participant presented with MCI or depression and when the dementia was other than AD or FTD. MCI was associated with significantly longer time to family awareness of dementia diagnosis. Conclusions: Factors impacting on time to diagnosis vary with the stage of diagnosis in YOD. Longer time to dementia diagnosis occurred in people who were younger at symptom onset, when MCI or depression was present, and in dementias other than AD and FTD.