The longstanding question of how stochastic behaviour arises from deterministic Hamiltonian dynamics is of great importance, and any truly holistic theory must be capable of describing this transition. Using the Koopman von-Neumann formulation of classical mechanics, we present here the Classical Influence Functional (CIF) -a method to bridge the gap between deterministic and stochastic dynamics, and unify the formalisms describing these disparate behaviours. Using this new technique, we demonstrate how irreversible behaviour arises generically from the reduced microscopic dynamics of a system-environment amalgam. The classical influence functional is then used to rigorously derive a generalised Langevin equation from a microscopic Hamiltonian. In this method stochastic terms are not identified heuristically, but instead arise from an exact mapping only available in the path-integral formalism of classical mechanics. As a consequence of the CIF, we are also able to show that the proper classical limit of stochastic quantum dynamics corresponds to this Langevin equation, providing a further unifying link to quantum theories. These derivations highlight the utility of CIFs, and its potential as a tool in both fundamental and applied research.