  The Kohlrausch(-Williams-Watts) law of stretched exponential relaxation has
been observed for more than a century and a half in diverse complex classical
systems. Here we show that this law describes relaxation quite generically in
closed (executing Schr\"{o}dinger dynamics), interacting disordered many-body
systems across a range of system sizes using interaction range and disorder
strength as primary tuning parameters. This we observe for both
time-independent and periodically driven (Floquet) systems. Finite-size
analysis indicates the persistence of this non-thermal relaxation regime in the
thermodynamic limit thus defining a distinct dynamical regime. This regime
exhibits a peak in the time-scale of the perceptible relaxation, upon crossing
over from weak to strong disorder. We provide a simple picture of this
behavior, which naturally accounts for its general occurrence. Formation of
spin-glass -- one of the possible mechanisms for stretched relaxation appears
incidental to the occurrence of Kohlrausch law in our context. Finally, we
provide a simple non-Hermitian Hamiltonian formulation for the dynamics of a
single spin embedded in the disordered chain. This provides an analytical
formula that captures not only the Kohlrausch relaxation of the disorder
averaged auto-correlation but also captures the largely diverse dynamics of an
arbitrary target spin in the system. Our work hence also provides a concrete
quantification of the ``pre-thermal slowness" in many-body disordered system.
