Epitaxial thin films of ordered double perovskite ${\text{La}}_{2}{\text{NiMnO}}_{6}$ have been studied by a combination of high-resolution scanning transmission electron microscopy, quantitative electron energy loss spectroscopy, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD) spectroscopy. Our results show the nominal oxidation states of Ni and Mn ions to be ${\text{Ni}}^{2+}$ and ${\text{Mn}}^{4+}$ thus the ferromagnetism in ground state is mainly due to ${\text{Ni}}^{2+}{\text{-O-Mn}}^{4+}$ superexchange interactions. In addition, short-range ferromagnetic correlations are observed above the Curie temperature $({T}_{C}\ensuremath{\sim}280\text{ }\text{K})$ from XMCD measurement, which are likely induced by antisite defects against long-range ordering of the Ni/Mn sublattice. The XMCD results also demonstrate that the ${\text{Ni}}^{2+}$ and ${\text{Mn}}^{4+}$ ions are ferromagnetically aligned but exhibit large differences in the spin and orbital contributions to their effective magnetic moments.