The synthesis, crystal structure and magnetic properties of Ba1.2Mn8O16, obtained using a KCl mineralizer, are reported. This material crystallizes in the monoclinic hollandite structure, containing Mn-O double chains (space group: 12/m, a = 10.052(1) angstrom, b = 2.8579(2) angstrom, c = 9.7627(10) angstrom, beta = 89.96(1)degrees), and undergoes a magnetic transition at 40 K with the magnetization having a small ferromagnetic component. The ordering temperature is nearly 10 times lower than expected from the Weiss temperature of -385 K, evidencing magnetic frustration. Preliminary neutron powder diffraction studies suggest a complex spin arrangement within the ac-plane (perpendicular to the chain axis). The resistivity is consistent with one-dimensional variable range hopping.