Global aspects of classical and axiomatic potential theory are developed in a purely algebraic way, in terms of a new algebraic structure called a mixed lattice semigroup. This generalizes the notion of a Riesz space (vector lattice) by replacing the usual symmetrical lower and upper envelopes by unsymmetrical "mixed" lower and upper envelopes, formed relative to specific order on the first element and initial order on the second. The treatment makes essential use of a calculus of mixed envelopes, in which the main formulas and inequalities are derived through the use of certain semigroups of nonlinear operators. Techniques based on these operator semigroups are new even in the classical setting.