The Mountain Defense League Records represents the administrative records of the Mountain Defense League (MDL) for its three decades existence, and the work of both Bryon Lindsley and Pandora Rose as former presidents of the MDL. The majority of the collection consists of records from a variety of federal, state, and private organizations including legal documents, legislation, newsletters, annotated maps, and publications.  The MDL worked in conjunction with several federal agencies, most notably the Department of the Interior, U.S. Department of Agriculture, U.S. Forest Service, and the Bureau of Land Management. The collection is divided into two major series: Administrative Files and Environmental Research Files. The Administrative Files series is divided into five subseries: General Office Files, Correspondence, Legal Files, San Diego Environmental Review Board, and Sierra Club, San Diego Chapter.  The Environmental Research Files series is divided into five sub-series: Local Environmental Projects, General Plans, National Forests, Soil Surveys, and Publications.

The Administrative Files document the work of the Mountain Defense league from its inception in 1973 to the conclusion of Lindsley's presidency in 2002. The series is arranged in five subseries. The subseries contain development and fundraising work, general administrative documents, and legal files. The series also includes a section on the San Diego chapter of the Sierra Club which Lindsley was a member.

The Environmental Research Files document project files and regional development areas focused on environmental impacts and studies. The series is divided into five subject-based sub-series. Also included is the documentation of the MDL's decision making process regarding its focus and project selection. The first sub-series, Local Environmental Projects, documents several environmental issues in San Diego county from the 1970s to early in the 21st century, and consists predominantly of Environment Impact Reports (EIR), Environmental Impact Statements (EIS), biological or environmental assessments, management plans, legal action files relating to a specific topic within the project files, and records of decision for a few of the EIRs. It is arranged in alphabetical order based on the subject of documents. The second sub-series, General Plans, contain sections of the San Diego County General Plan. This sub-series also contains EIRs and legal action files pertinent to a specific portion of the General Plan are arranged alphabetically. The third sub-series, National Forests, largely document the MDL's work regarding the four major national forests in and around San Diego County: Angeles, Cleveland, Los Padres, and San Bernardino. It is arranged alphabetically. The fourth sub-series, Soil Surveys, documents the potential development area's soil contents. It is arranged in numerical order corresponding to the map number listed on each soil survey map with the exception of the two volume bound text that precedes the maps in the sub-series arrangement. The fifth and final sub-series, Publications, document a wide range of conservation and environmental topics. Materials in this section include government documents, wildlife magazines, brochures, news articles, federal registers, a census of agriculture, and legislation.