FGF21 contributes to the metabolic response to dietary protein restriction, and prior data implicate GCN2 as the amino acid sensor linking protein restriction to FGF21 induction. Here, we demonstrate the persistent and essential role of FGF21 in the metabolic response to protein restriction. We show that Fgf21 KO mice are fully resistant to low protein (LP)-induced changes in food intake, energy expenditure (EE), body weight gain, and metabolic gene expression for 6 months. Gcn2 KO mice recapitulate this phenotype, but LP-induced effects on food intake, EE, and body weight subsequently begin to appear after 14 days on diet. We show that this delayed emergence of LP-induced metabolic effects in Gcn2 KO mice coincides with a delayed but progressive increase of hepatic Fgf21 expression and blood FGF21 concentrations over time. These data indicate that FGF21 is essential for the metabolic response to protein restriction but that GCN2 is only transiently required for LP-induced FGF21.