We report on the first synthesis of Li-intercalated manganese-phthalocyanine (MnPc) in the bulk form and on the evolution of the structural and magnetic properties as a function of Li concentration, x. We find that solid beta-MnPc, which comprises rodlike assemblies of individual planar molecules, is best described as a glassy one-dimensional ferromagnet without three-dimensional ordering and that it can be quasi-continuously intercalated with Li up to x= 4, forming an isosymmetrical series of Li-x[MnPc] phases. Inserted Li+ ions strongly bond to pyrrole-bridging nitrogen atoms of the PC rings, thereby disrupting the ferromagnetic Mn-(NaMn)-Mn-... superexchange pathways. This gradually induces a crossover of the intrachain exchange interactions from ferromagnetic to antiferromagnetic as the doping level, x, increases coupled with a spin-state transition of the Mn2+ ions from intermediate spin, S = 3/2, to high spin, S = 5/2.