We study systems of functional equations whose solutions can be parameterized in function of one variable; our main result proves that the partition regularity (PR) of such systems can be completely characterized by the existence of constant solutions. As applications of this result, we prove the following: (1) A complete characterization of the PR of systems of Diophantine equations in two variables over $\mathbb{N}$. In particular, we prove that the only infinitely PR irreducible equation in two variables is $x=y$; (2) PR of $S$-unit equations and the failure of Rado's Theorem for finitely generated multiplicative subgroups of $\mathbb{C}$; and (3) a complete characterization of the PR of two classes of polynomial exponential equations.