Al-Doped magnetite spinel nanoparticles encapsulated in mesoporous carbon (MC) were identified as a promising heterogeneous Fenton catalyst towards phenol degradation in a continuous system for practical applications. Under the working conditions inside the fixed-bed reactor hercynite in the fabricated 21%γ-Fe2O3/28%FeAl2O4@MC material reacts with H2O2. In this reaction Al ions occupied the vacant octahedral cationic sites in the framework of the γ-Fe2O3component converting it to Al substituted magnetite spinel. The Al in the obtained Fe3+0.66Fe2+0.33(Fe2+0.33Fe3+0.33Al3+0.33)2O4@MC polarizes electrons of iron ions through its Lewis acid properties imparting more positive charge on iron ions (Fen+(δ+)). This expedites the rate of the challenging reduction reaction Fe3+→ Fe2+with H2O2to produce HOO˙ and reinforces the bonding of iron ions in the spinel improving their activity and stability. Hence, under mild operating conditions (pH 5, 40 °C, 8.6 ml water per ml cat h−1, 0.036 mol H2O2, 200 ppm phenol) thein situproduced catalyst Fe(Fe0.66Al0.33)2O4@MC of 35 nm containing 19.9%Fe and 2.4%Al with a surface area of 335 m2g−1exhibited long-lasting high catalytic activity and stability during a 500 h run. 80% TOC conversion and ∼1 ppm of leached Fe in treated water were obtained without a visible change of catalytic performance.