This study focuses on the mineralization of 5-amino-6-methyl-2-benzimidazolone (AMBI), an important precursor in the industrial production of dyes, through an integrated Fe(III) photoassisted–biological system without addition of other electron acceptor than O2. The iron photoassisted process produces a biocompatible solution, removing 100% of the initial biorecalcitrant compound and 40% of the Dissolved Organic Carbon (DOC), and then the complete mineralization was achieved in the biological treatment. The transformation of AMBI photoinduced by only Fe(III) is mainly attributed to three simultaneous process: direct photolysis of the [Fe3+–AMBI] complex, the attack of the complex by √OH radicals generated by the photolysis of Fe(OH)2+, and by the attack of supplementary √OH radicals generated by the Fenton and photo-Fenton like reactions, which are induced by the H2O2 that have been formed “in situ”. The following topics are also studied in this paper: (a) the UV-Vis spectroscopic characterization of Fe(III), AMBI, and its mixture in aqueous solution, (b) the involvement of √OH radicals and oxygen in the photodegradation of AMBI, (c) the comparison between O2 and H2O2 as electron acceptors in the iron photoassisted pretreatment, and their performances as a pretreatment step in the coupled photochemical biological reactor. Some field experiments under direct solar radiation were carried out using a compound parabolic collector (CPC) erected at the Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland. The obtained results indicate that a coupled solar–biological treatment system at pilot scale is a possible way to achieve the complete mineralization of biorecalcitrant pollutants.