  Monte Carlo event generators are a critical tool for the interpretation of
data obtained by neutrino experiments. Several modern event generators are
available which are well-suited to the GeV energy scale used in studies of
accelerator neutrinos. However, theoretical modeling differences make their
immediate application to lower energies difficult. In this paper, I present a
new event generator, MARLEY, which is designed to better address the simulation
needs of the low-energy (tens of MeV and below) neutrino community. The code is
written in C++14 with an optional interface to the popular ROOT data analysis
framework. The current release of MARLEY (version 1.2.0) emphasizes simulations
of the reaction $^{40}$Ar$(\nu_e, e^{-})^{40}$K$^{*}$ but is extensible to
other channels with suitable user input. This paper provides detailed
documentation of MARLEY's implementation and usage, including guidance on how
generated events may be analyzed and how MARLEY may be interfaced with external
codes such as Geant4. Further information about MARLEY is available on the
official website at http://www.marleygen.org.
