Double perovskite La2CoMnO6 is an attractive spintronics with magnetoelectric, magnetodielectric, and magnetoresistive effects, which are related, at least in part, to combined structural and magnetic instabilities. To explore its magnetoelastic coupling behaviour, a conventional analysis of lattice parameter data in terms of spontaneous strain shows that the ferromagnetic ordering process is accompanied by significant volume (e(aM)) and shear (e(tM)) strains. The DC and AC magnetic data reveal the antisite defects with antiphase regions, and the ferromagnetic transition is at 230K with a small antiferromagnetic ordering at about 170K. In addition, the Rietveld refinement of the in situ variation X-ray diffraction from 120 to 1400K, scanning electron microscopy, and differential scanning calorimetry were used to confirm the crystal structure, microstructure, magnetic, and structural anomalies of the sample synthesised in pure oxygen atmosphere. This study, and in particular the strain analysis, of La2CoMnO6 will facilitate its potential application in the field of spin electronics and thin-film engineering.