The charge-transfer (HMTTF)[Ni(mnt)2] material (1) was the first mixed-stack compound reported to present ferromagnetic (FM) interactions between the donor (D) and acceptor (A) units within each mixed-stack. Despite the presence of a dominant FM interaction, its magnetic susceptibility curve, [small chi](T), corresponded to that of an antiferromagnetic (AFM) compound at low temperatures, fact that was tentatively explained in terms of a FM-to-AFM magnetic transition. In this work, the First-Principles Bottom-Up procedure has been applied to rationalize the magnetic properties of 1. The results presented herein indicate the presence of up to five sizeable JAB magnetic exchange interactions (26.0, -1.7, 1.9, 0.1, and -99.3 cm-1) that create a complex 2D magnetic network. This magnetic topology not only produces a calculated [small chi](T) curve that is in excellent agreement with the experimental measurements, but also allows us to understand the magnetic behaviour of 1, at the full range of temperatures