Endogenous lectins play key roles in cell homeostasis by decoding the information encrypted in glycans present on the cell surface or extracellular matrix. Galectins, a family of soluble lectins, have emerged as central regulators of innate and adaptive immune responses. In this article, we review seminal work demonstrating the immunoregulatory roles of Galectin-1 (Gal-1), a proto-type member of the galectin family, and highlight central mechanisms that control its functions in cancer and autoimmune inflammation. Understanding the cellular pathways that control Gal-1 expression and function in tumor and inflammatory microenvironments will set the bases for the design of rational therapies based on positive or negative modulation of this endogenous lectin in cancer and autoimmune diseases.