The structural, electronic and magnetic properties of anti-site disordered Sm 2 NiMnO 6 double perovskite has been studied. RE 2 NiMnO 6 (RE: rare-earth) ordered double perovskite is commonly believed to show two distinct magnetic phase transitions viz, paramagnetic to ferromagnetic (FM) transition at T = T C due to Ni-O-Mn super exchange interaction and another transition at T = T d due to coupling of RE spins with Ni-Mn network. In our present study, we have observed that the presence of intrinsic B-site disorder results in an additional antiferromagnetic (AFM) coupling, mediated via Ni-O-Ni and Mn-O-Mn local bond pairs. As a consequence, the magnetic behavior of SNMO comprises of co-existing FM-AFM phases, which are respectively governed by the anti-site ordered and disordered structures. Field dependent inverted cusp like trend in M(T) and two step reversible loop behavior in M(H) measurements indicate the presence of competing FM-AFM phases over a wide range of temperature values (T d < T < T C ).