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One‐third of the world's trees are at risk of extinction, with large, old, long‐lived trees among the most vulnerable. Long‐lived trees in arid and semi‐arid biomes are particularly at risk, including Australian sandalwood (Santalum spicatum, Santalaceae), which is experiencing substantial population decline due to a suite of natural and anthropogenic drivers, with no appreciable recruitment estimated for more than 80 years. To contextualize this range‐wide collapse and quantify regional variation in population dynamics across Australia's western rangelands, we investigated the size‐class profiles of 12 sandalwood populations in a 1,500‐kilometre arc between Shark Bay and the Gibson Desert in central Western Australia including Indigenous Protected Areas, pastoral leases and public and private conservation parks and reserves. Stem diameters, indicative of age using known growth rates, were recorded for 1,355 sandalwood plants, along with a set of another plant structural and ecological parameters. Using size‐class profiles and associated demographic data, we estimated the population age structure and trajectory to determine whether each population was increasing, stable or declining. Our surveys revealed sandalwood populations are declining and are composed almost entirely of very old trees in advanced states of senescence. Of 1,355 plants sampled, 1,198 (88.4%) individuals were large (old) trees. A total of 23 seedlings and 21 saplings were recorded across all sites, almost all of which (22 and 19, respectively) were in one population, and located under the canopies of parent trees where they would not be expected to survive to maturity. Our findings reinforce the urgent need to list Santalum spicatum as a threatened species in Western Australia (where wild plants are still being commercially harvested) and to initiate effective conservation actions to secure the species' continued existence across its natural range. [ABSTRACT FROM AUTHOR]