Trivia: I once worked with her daughter, Dr. Catherine Bateson, in the Sociology Department at Northeastern.
I searched high and low for the source. That is when I found this information on the website of the Institute for Interculture Studies -- a group which trademarked the quote :
What is the source of the “Never doubt...” quote?
Although the Institute has received many inquiries about this famous admonition by Margaret Mead, we have been unable to locate when and where it was first cited, becoming a motto for many organizations and movements. We believe it probably came into circulation through a newspaper report of something said spontaneously and informally. We know, however, that it was firmly rooted in her professional work and that it reflected a conviction that she expressed often, in different contexts and phrasings.
Mead’s fullest exploration of cultural transmission and of the place of small groups in cultural change and innovation was in Continuities in Cultural Evolution, first published in 1964. Appropriately, the reissue of this book was one of the first goals of the Mead2001 celebration, and it has now been brought out by Transaction Publishers with a new introduction by Stephen Toulmin setting it in the context of contemporary evolutionary thinking.
The "Never doubt" challenge is sometimes quoted in a longer form, with the coda, "indeed, it's the only thing that ever has." We decided on the shortened form both for brevity and because the exaggeration in the coda may, in print, weaken the basic concept rather than reinforce it.
P.S. It has been my experience when you cannot source a quote, it was never uttered by the person. That said, here are some popular variations I located:
Never doubt that a small group of thoughtful, committed people can change the world. Indeed, it is the only thing that ever has.
Never believe that a few caring people can't change the world. For, indeed, that's all who ever have.
A small group of thoughtful people could change the world. Indeed, it's the only thing that ever has.
Never doubt that a small group of thoughtful, committed citizens can change the world; indeed, it's the only thing that ever does.