First principles calculations based on the generalized gradient approximation to the density functional theory reveal the existence of two nearly degenerate isomeric forms of Mn13 cluster. The lowest energy structure corresponds to icosahedric packing that lies 0.65 eV lower than a caged structure with a hexagonal close-packed motif. Magnetically, however, these two isomers are very different: while the icosahedric structure is ferrimagnetic with a total magnetic moment of 33 μB, the hexagonal close-packed structure is a de facto antiferromagnet with a magnetic moment of 17 μB. The two isomers also have different ionization potentials, 5.19 and 4.82 eV, in good agreement with experiment.