[1] He initiated many organizations, the most prominent of which are the Maine Green Party (1984); the Green Party of the United States (1984–87) for both of which he was a principal founder;[2] and the Cathance River Education Alliance (2000), a hands-on ecological education project for local schools, schoolchildren and high school students in mid-coast Maine.

His mother, Effie (Aafje Kooiman), was born in the Netherlands; his father, John Rensenbrink, was the eldest son of Dutch immigrants.

Rensenbrink studied history, English and philosophy at Calvin and was editor of the college newspaper during his junior and senior years.

[2] As he prepared for his first class at Williams in the summer of 1957, Rensenbrink met Carla Washburne in her father's college bookstore in Williamstown.

He and Carla and their daughters Kathryn and Margaret, (born in Dar es Salaam), aged three and one respectively, returned to Bowdoin College in 1965.

Eluding the watchful eye of the Communist regime's secret police, he researched and studied the sources and shape of social change as represented by Solidarity.

He wrote his first book, based on that experience, in 1988, published by the University of Louisiana Press, "Poland Challenges a Divided World."

Following semi-retirement in 1989, Rensenbrink continued teaching at Bowdoin for several years, creating an interdisciplinary seminar for majors in Black, Women's, and Environmental Studies.

Shortly after, at the University of Michigan in 1951–52, he joined the Young Republicans, but found himself disgusted with the politics of Joseph McCarthy.

The campaigns were battles lost, by close margins, but the struggle against nuclear power was won in terms of public opinion.

That summer, on his way back to the United States, Rensenbrink stopped off in Munich and in Frankfurt to visit friends who had joined the German Green Party and were celebrating their unexpected parliamentary success.

[7] Rensenbrink quickly made plans to seek early retirement (which was accomplished in 1989) and threw himself into Green Party organizing in Maine and in the United States.

Working with the Clearing House, the annual gathering of a Green Assembly, and the Inter-Regional Committee that had formed, Rensenbrink headed a three-year project to produce a Green Policy Program, generated from the over 300 grass roots groups that had sprung up in the early years.

The Program was completed and approved in September 1990, at the annual meeting of the Green Assembly in Boulder, Colorado.

In 1999, Rensenbrink, together with his wife Carla and several fellow townspeople, created Topsham's Future, a citizen action group dedicated to balancing the then very rapid economic development of Topsham with the preservation and vitality of community values and neighborhood integrity.

One of its major accomplishments was creation of the Cathance River Nature Preserve through extended negotiations with retirement-community developer John Wasileski.

Building on their success in the negotiation, Rensenbrink and Wasileski joined together to found the Cathance River Education Alliance a program in 2000.

This committee, chaired by Rensenbrink, produced a Report recommending improvements in Topsham's Town Meeting, some of which have been instituted with others pending.