Climate change affects the Mediterranean region stressing lentil crops during flowering and seed set. Early maturation and drought tolerance are desirable traits in these conditions. Phosphorus (P) is considered to enhance early flowering, maturity and thus yields. Four P rates (0, 30, 60, 90 kg P2O5 ha−1) were applied on four cultivars (Samos, Thessaly, Flip, Ikaria) during two seasons. Growing degree-days (GDD) were calculated for vegetative (V4–5, V7–8) and reproductive stages (R1, R2, R4, R6, R8). At R2 (full bloom) carbon isotope discrimination (Δ) was used to assess water-use efficiency. At R8 (full maturity), the seed weight (SW) was determined by harvest. Cultivars, P and the P × cultivar and P × growth season interactions affected the earliness in reproductive stages; P had no effect on GDD of vegetative stages. Phosphorus both induced earliness (Flip, Thessaly) and delayed maturity (Samos, Ikaria). GDD and SW were negatively correlated for the P × cultivar interaction at R1 (first bloom), R2, R4 (flat pod) and R6 (full pod filling) stages; being the strongest at R1. Negative correlations were evident for the P × growth season interaction at R2, R4 and R6 stages; being the strongest at R4. Cultivars and P did not affect Δ. A proper combination of cultivar and P rate can mitigate lentil yield losses under changing Mediterranean climate.