The mammary gland is a unique organ that develops predominantly after birth and undergoes dramatic remodelling in reproductive phases. The immune system is intimately tied to mammary gland function and disease, with key roles in morphogenesis, tissue remodelling, pathogen clearance and cancer. Although macrophages have been implicated in mammary gland function and remodelling, their diversity has not been fully addressed. Through whole organ-clearing and high-resolution 3D imaging, we have identified a unique population of dendritic-shaped tissue-resident ductal macrophages (DMs) that form a contiguous network between the luminal and basal layers of the entire mammary gland throughout post-natal development. The distinct localisation of DMs allowed characterisation of marker expression and subsequent isolation by flow cytometry. We show that DMs are uniquely dependent on the epithelium and are long-lived tissue-resident macrophages. While they initially originate from embryonic precursors, DMs are generated from monocytes as they increase in parallel with the epithelium during puberty. We observed DM interaction with the epithelium using a novel intravital imaging approach. This revealed that DMs do not migrate but constantly survey the epithelium through dendrite movement. DMs undergo proliferation in pregnancy to maintain complete coverage of the epithelium in lactation, where they are poised to phagocytose milk-producing cells post-lactation and facilitate remodelling. DMs strongly resemble mammary tumour-associated macrophages and increase throughout tumorigenesis to form a network that pervades the tumour epithelium. Thus, the mammary epithelium programs specialised resident macrophages in both physiological and tumorigenic contexts. This knowledge of a new breast macrophage population will help future studies on how breast-resident immune cells respond to infection and cancer.