The ferromagnetic properties of the double perovskite La₂NiMnO₆ are governed by the Ni²⁺–O²⁻–Mn⁴⁺ superexchange interaction. The Ni²⁺ and Mn⁴⁺ valence states along with the antisite disorder strongly influence the magnetic interactions in the compound. Here, we report on the effect of high temperature annealing on the antisite disorder and related properties of La₂NiMnO₆. X-ray absorption measurements reveal nominal changes in the local environment around B/B′ sites of all samples. However, saturation magnetization decreases due to increased antisite disorder, which enhances the Ni²⁺–O²⁻–Ni²⁺ and Mn⁴⁺–O²⁻–Mn⁴⁺ antiferromagnetic exchange interactions. At 5 K, a large saturation moment of ∼4.95 μ_B/f.u. is observed for the sample annealed at 950 °C. For all samples, the ferromagnetic–paramagnetic transition is observed at 280 K, which is attributed to the fact that the Ni–Mn interatomic distance remains constant for all the samples. Our study suggests that the Ni/ Mn valency and Curie temperature are insensitive to the antisite disorder present in the system.