Phenols’ valorisation via step- and atom-efficient protocols is of great importance for the transition to sustainable chemical production. Among the transformation of phenols, C–H functionalization is limited to a few examples with homogeneous metal-based catalysts. Herein, a robust one-pot approach for constructing C8 alkyl substituted coumarins starting from simple phenols through the palladium-catalysed double C–H functionalizations is presented. In such a combined protocol, a Pd/C was used as a heterogeneous catalyst for the first time. The developed methodology enables the synthesis of structurally diverse coumarins, including natural products, in good isolated yields using potentially biomass-derived phenols. Several TEM and XPS measurements combined with elemental analysis were conducted in order to gain insight into the nature of the active catalyst and the factors affecting its reactivity.