Nanosized, rectangularly arranged ZnO–CeO$_2$ nanoflowers were prepared by a simple hydrothermal technique. The prepared nanocomposites were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), UV–visible differential reflectance spectroscopy, field-emission scanning electron microscope, X-ray photoelectron spectroscopy, electron paramagnetic resonance and Brunauer-Emmett-Teller techniques. Different compositions of composites were prepared and the best composition was used for conducting the photocatalytic studies. The degradation of 2,4-dichlorophenoxy acetic acid (2,4-D), a herbicide and endocrine-disrupting agent, was performed under visible-light irradiation. The results showed the nanocomposite to be in a flower-like structure comprising of rectangular-shaped ZnO on which CeO$_2$ particles were evenly distributed. This flower-shaped ZnO–CeO$_2$ nanorectangulate composite comprises particle sizes ranging from 50–100 nm of rectangular thickness and 10–20 nm spherical particles. The XRD and FT-IR results prove the presence of ZnO and CeO$_2$ in the prepared nanocomposite. The bandgap energy was found to be 3.18 eV. X-ray photoelectron spectroscopy proved the presence of Ce as Ce$^{4+}$ (CeO$_2$) in the prepared composite. A comparative study of nanocomposite and pristine ZnO showed increased activity. The photocatalytic preliminary and kinetic studies were conducted. The reaction followed pseudo-first-order kinetics. The UV–visible absorption spectrum and chemical oxygen demand decrease proved the complete degradation of the endocrine disruptor 2,4-D under visible-light irradiation.