Background: Seriously ill individuals rely heavily on family caregivers at the end of life. Yet many do not have family support. Aim: To characterize the size and composition of decedents’ family networks by cause of death, demographic, clinical, socioeconomic, and geographic characteristics. Design: A cross-sectional population-level study with data collected from nation-wide registers. Setting/participants: All adults in Denmark born between 1935 and 1998 who died of natural causes between 2009 and 2016 were linked at the time of death to living adult spouses/partners, children, siblings, parents, and grandchildren. Results: Among 175,755 decedents (median age: 68 years, range: 18–81 years), 61% had a partner at the time of death and 78% had at least one adult child. Ten percent of decedents had no identified living adult family members. Decedents with family had a median of five relatives. Males were more likely to have a spouse/partner (65%) than females (56%). While 93% of decedents dying of cancer had adult family, only 70% of individuals dying of dementia had adult family at the time of death. The majority of cancer decedents co-resided or lived within 30 km of family (88%), compared to only 65% of those dying from psychiatric illness. Conclusions: While the majority of adults had an extensive family network at the time of death, a substantial proportion of decedents had no family, suggesting the need for non-family based long-term service and support systems. Assessment of family networks can expand our understanding of the end-of-life caregiving process and inform palliative care delivery.