Peace Brigades International was founded in 1981 at a conference of people from across the globe with a history in non-violent action, including Daniel Clark, held on Grindstone Island, Canada. The secretariat of Peace Brigades International was originally stationed out of Clark's home office, where he held the position of International Secretary and legal consultant for the organization's initial years, until the secretariat was moved to Boston, Washington D.C., London, and finally to its current location in Brussels. Clark has continued as a legal advisor for Peace Brigades International. This series contains legal and organizational records of Peace Brigades International from 1980 to 2020.

This series contains records dating from 1977 to 2000 relate to the non-violent civil disobedience performed at the Hanford site throughout the 1980s. As Daniel Clark acted as a volunteer attorney for several of the protestors, this series contains legal records, newspaper clippings, and conference material on nuclear waste management and impacts.

This series contains records related to Daniel Clark's work around criminal justice reform from 1983 to 2016, primarily as an attorney for various individuals and organizations, including STAR, the Successful Reentry and Transition Project, a rehabilitation organization collaborating with the Washington State Penitentiary and the Department of Corrections. This series includes a variety of organizational records, protest records, and criminal justice reform legislation.

This series contains records that date from 1992 to 2017 related to Alliance for the Homeless, an organization founded by Clark in 2015, and the Getting off the Road campaign, which provided a location, community, and legal aid for homeless individuals from 2016 to 2017. The series also contains records related to the development of a permanent shelter (Madison Park), as well as records of smaller programs such as laundry and toilet access. This series includes publicity, volunteer reports, organizational records, legal records, and records relating to similar programs in other cities.

Community-to-Community (formerly known as the Sister City Hunger Project) was established by Dan Clark in 1980. The organization served to connect communities in North America with communities in Central America in order to generate increased income and work opportunities. In November of 1980, Clark traveled to Central America and Mexico to do relational work and build connections through which he could develop these relationships. Clark's involvement with Community-to-Community continued for eleven years until 1991, when he turned the organization over to Scott Renfro and others.

This series contains records related to Dan Clark's work on a public response opposing a carbon sequestration project that ran from 2006 to 2009, and related plans for a coal-fired power plant at Wallula. The sequestration project was a collaboration between several companies and institutions, including Batelle and Montana State University. The series also includes the records of several Freedom of Information Act requests related to State of Washington policies on carbon production and management.

This series includes materials related to the economic growth and land management needs of Walla Walla from 1989 to 2007. Dan Clark was a strong proponent of local businesses and community development through local purchasing, as well as the Growth Management Act, which requires fast-growing communities to develop contingencies for population growth. This series also includes files relating to land use appeals from agricultural land owners.

The Bike and Pedestrian series includes materials related to infrastructure development for bicycle and pedestrian traffic in Walla Walla from 1990 to 1996, as well as several events intended to popularize alternative transportation methods. It also includes meeting minutes and other materials produced by the City Bicycle Advisory Committee to the Walla Walla City Council.

The Valley Transit series includes materials related to the campaign to maintain the existing bus schedule during the 2009 local election cycle. This includes advertisements, materials from the Valley Transit Board, and instructions for each prong of the campaign strategy (such as phoning and voter registration). These materials all date from 2009.

The Walla Walla Diversity Coalition aims to present the history of various racial and ethnic groups that reside in Walla Walla. This series contains newspaper clippings, photographs, correspondence, interviews, and biographical information, dating from 1967 to 2022.

The Don't Bend Walla Walla series includes records regarding opposition to the Pennbrook Homes' Illahee Project. The Illahee Project's goal was to build upwards of 300 homes, a golf course, hotel, and restaurants in anticipation of a population increase, which caused tension among Walla Walla citizens who wanted it to remain a small town. The series contains newspaper clippings, compact discs, interview notes, statements from Barbara Clark, legal documents and other planning documents, dating from 1993 to 2007.

Walla Walla 2020 (abbreviated to WW2020) is a group of locals dedicated to the maintenance of the environment and communal spaces in and around Walla Walla. Barbara and Daniel Clark were founding members of WW2020, with the organization tracing its roots back to a workshop Barbara held in 1988. This series contains newspaper clippings, press releases, certificates, correspondence, and brochures between the years of 1988 and 2015.

Daniel and Barbara Clark founded Denver Reconciliation Services in 1978. Soon after, Daniel would turn over leadership of Denver Conciliation Services (which would later change its name to Center of Dispute Resolution) and move back to Walla Walla. In April of 1979, Daniel and Barbara decided to create Neutral Ground, a non-profit mediation service. According to Daniel Clark, Neutral Ground is "the oldest general mediation program in the state of Washington." This series contains brochures, legislation, legal forms, meeting minutes, business, strategy, development plans, and school presentation materials, dating from 1977 to 2014.

The Society of Friends series is composed of records and materials for informational or political events from the Society of Friends ranging from 1968 to 2024. Many of the materials regard events made to advocate for anti-war and in opposition to violent conflict. Many of the materials are in the form of Union-Bulletin publications, event flyers, informational guides, meeting correspondence and letters, articles, pamphlets, poems, songs, and newsletters. Much of the correspondence and informational materials regard the Committee on Nonviolent Resolution. Many articles are from the Union-Bulletin which have been quoted in Daniel Clark's blog "The Unitive Self" where he writes about his personal philosophy and thoughts on spirituality.