The present work demonstrates the field emission characteristics and photocatalytic behavior of ZnCo₂O₄ marigold flowers synthesized via a facile hydrothermal method. The effect of annealing of these 3D porous hierarchical nanostructures on field emission and photocatalytic performances is studied. When compared with the as-synthesized sample, annealed ZnCo₂O₄ exhibits a ~2-fold improvement in photocatalytic activity for methylene blue (MB) degradation under visible light irradiation. The turn-on, threshold fields and high emission current densities are also strongly influenced as a result of annealing. The turn on field required to draw an emission current density of ~1 μA/cm² is found to be ~2.4 and ~1.8 V/μm for as-synthesized and annealed ZnCo₂O₄ marigold flowers, respectively. Field-emission measurements demonstrate remarkably large field enhancement and better stability for annealed samples. The X-ray diffraction and Raman analysis reveal that annealing improves the crystallinity and also help to remove the structural defects in ZnCo₂O₄. The enhancement in the field emission and photocatalytic activity of annealed ZnCo₂O₄ marigold flowers is attributed to the modification of electronic properties as a result of dehydration, crystallite growth and reduced surface defects/impurity phases.