Phalaris aquatica L. is a cool-season perennial grass of the Mediterranean that has been used for forage production, while recently was identified as a novel candidate for sustainable second-generation bioethanol production. This study provides for the first time evidence about this energy crop long-term performance (five years of growth), the cell wall saccharification efficiency and the production of high added value compounds during this period. High biomass production was recorded constantly after the first year of plant growth, with a maximum of 22.4 ± 0.5 t of dry mass (DM) per ha and stable structural polysaccharides concentration 68.8% ± 0.8 w/w DM from the third year on. Independently from crop age, dilute acid pretreatment (120 °C, 1.5% H2SO4, 45 min) promoted the subsequent enzymatic hydrolysis of P. aquatica L. biomass. The dilute acid pre-treatment positive effect is attributed both to the hemicellulose solubilization and the breakage of intermolecular bonds between polysaccharides and phenolic acids, as supported by the present study results These combined acid/enzymatic hydrolysis processes were able to convert more than 80% of cell wall polysaccharides into fermentable sugars, releasing simultaneously a substantial amount of phenolic acids of 2.16% ± 0.48 w/w DM. The ratio of ferulic to p-coumaric acid in the acid hydrolysate can be used for the prediction of the subsequent cellulose enzymatic saccharification efficiency, by the model described here. The protocatechuic acid, which can find many technological applications, was the main bioactive polyphenol in the final hydrolysate with concentration ∼1% w/w DM. These results are supportive of Phalaris aquatica L. suitability as a sustainable biorefinery crop, under the semi-arid Mediterranean climatic conditions contributing to this region biodiversity and prosperity.