Mebarek made his official debut at the 2004 Summer Olympics in Athens, where he qualified for two swimming events. In the 400 m freestyle, he challenged seven other swimmers on the third heat, where South Korea's Park Tae-Hwan was disqualified for a false start. Mebarek edged out Slovenia's Bojan Zdešar to take a fourth spot and dip under a 4-minute barrier by 0.90 of a second in 3:59.10. In his second event, 200 m freestyle, Mebarek shared a thirty-fifth place tie with Macedonia's Aleksandar Malenko in the preliminaries with a time of 1:53.00.