<p>Markovianity and local detailed balance (LDB) are widely regarded as two basic structural assumptions of stochastic thermodynamics. In this work, we use microcanonical ensemble theory to establish these properties for a small Hamiltonian system that is strongly coupled to its environment, also modeled as a Hamiltonian system, under the following assumptions: (i) the bath dynamics is much faster than both the system dynamics and the variation of the control parameters, i.e. time-scale separation (TSS); (ii) the bath is much larger than the system; (iii) the interaction between the system and the bath is short-ranged; (iv) the microscopic dynamics of the joint system has time-reversal symmetry, and (v) the coarse-grained dynamics of the joint system is Markovian. Under these assumptions, the bath remains in instantaneous microcanonical equilibrium conditioned on the system state and the control parameter. We decompose the total Hamiltonian such that the bath Hamiltonian is an adiabatic invariant under slow evolution of the system state and control parameters, which enforces the system Hamiltonian to be the <i>Hamiltonian of mean force</i>. The heat absorbed by the system is identified as the negative of the bath’s Boltzmann entropy change multiplied by <i>T</i>. Our approach provides a thermodynamically consistent and experimentally testable foundation for strong-coupling stochastic thermodynamics.</p>