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A mixture of cobalt ferrite and magnesium ferrite was obtained from layered double hydroxides through a coprecitation method with a theoretical molar ratio M2+/Fe3+ = 3/1, where M2+ represents Fe2+, Mg2+ and/or Co2+. The synthesized samples were characterized by X-ray diffraction, N2 adsorption, UV–vis spectroscopy, inductively coupled plasma, optical emission spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction. The photocatalytic activity of the sample under UV-light (254 nm) was evaluated by the degradation of phenol and 2,4-dichlorophenoxyacetic acid (2,4-D). Materials with a high CoFe2O4 content exhibited high activity with a total organic carbon removal in 180 min above 90% for the photocatalytic degradation of 2,4-D and phenol. The high activity was discussed in terms of the specific areas and the phases present in the materials. Thus, the presence of Co2+ in the ferrite phase highly dispersed on a matrix of magnesium oxide with a low recombination rate and the efficient regeneration of the surface Fe3+ and Co2+ species explain the results obtained. The biodegradability index of the 2,4-D solution increased above 0.4 when materials modified with Co were irradiated for 180 min in the presence of H2O2. So, it was possible to convert nonbiodegradable 2,4-D solutions into biodegradable forms. The degradation intermediates such as 2,4-dichlorophenol, 4,6-dichlororesorcinol, 2-chlorohydroquinone, and some small-molecule acids were identified by gas chromatography/mass spectrometry. Moreover, CoFe2O4 on MgO showed good recycling performance and stability for the degradation of 2,4-D. Furthermore, a possible photocatalytic mechanism of degradation was proposed.