Mast cells are a heterogeneous population of immune effector cells derived from haematopoietic progenitors in the bone marrow that undergo final differentiation in peripheral tissues. Their strategic localisation within barrier tissues, such as the skin and the mucosal surfaces of the respiratory and gastrointestinal tracts, underlies their critical role in the initiation and modulation of immune responses. As key components of innate immunity, mast cells recognise pathogen-associated molecular patterns (PAMPs) through pattern recognition receptors (PRRs), leading to their activation and the release of pro-inflammatory mediators. Furthermore, mast cells exert important immunoregulatory functions through interactions with other immune cells, including T lymphocytes, dendritic cells, and macrophages, thereby shaping inflammatory processes and immune responses. Mast cells play a central role in the pathogenesis of allergic diseases, where they act as primary effector cells in IgE-mediated type I hypersensitivity reactions. Cross-linking of FcεRI receptors induces rapid activation and degranulation, resulting in the release of mediators responsible for the development of clinical symptoms. In contrast, in cancer, mast cells represent a key component of the tumour microenvironment (TME) and exhibit a context-dependent role in tumour progression. They may promote tumour development through the induction of angiogenesis, extracellular matrix (ECM) remodeling, and modulation of immune responses, but they can also contribute to anti-tumour immunity. This review summarises current evidence on shared molecular and cellular mechanisms of mast cell function in allergic diseases and cancer, with a focus on their clinical relevance as potential therapeutic targets.