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We have investigated the magnetic state of Mn_1/3NbS₂ through ac-magnetic susceptibility, Small-angle Neutron Scattering (SANS), Lorentz Transmission Electron Microscopy (LTEM) and micromagnetic simulations. The ac susceptibility displays temperature, field, and frequency dependencies which define a complex phase diagram below the critical temperature for magnetic ordering, T_C = 45 K. SANS reveals a streak of magnetic scattering along the c-axis near Q = 0 appearing below T_C, demonstrating a disordered ferromagnetic (FM) or helical spin ordering in this system. The width of this streak shortens and becomes more intense near T_C and is gradually suppressed by the application of H along the beam. Micromagnetic simulations of thin lamella are in agreement with LTEM images of Mn_1/3NbS₂ where extended FM regions result from a shape anisotropy in thin samples that are separated by the chiral domain walls.