This paper presents and summarizes a software package ("LPipe") for completely automated, end-to-end reduction of both bright and faint sources with the Low Resolution Imaging Spectrometer (LRIS) at Keck Observatory. It supports all gratings, grisms, and dichroics, and also reduces imaging observations, although it does not include multislit or polarimetric reduction capabilities at present. It is suitable for on-the-fly quicklook reductions at the telescope, for large-scale reductions of archival data sets, and (in many cases) for science-quality post-run reductions of PI data. To demonstrate its capabilities the pipeline is run in fully automated mode on all LRIS longslit data in the Keck Observatory Archive acquired during the 12-month period between 2016 August and 2017 July. The reduced spectra (of 675 single-object targets, totaling ∼200 hours of on-source integration time in each camera), and the pipeline itself, are made publicly available to the community.