Thomsen was born in 1915 into a wealthy American family in Seattle. Charlie, his father was President of Centennial Mills (Krusteaz Brand) and a multi-millionaire at the turn of the Century. As detailed in his memoirs, his relationship with his father was extremely strained, with Thomsen describing the man as "tyrannical."

During World War II, Thomsen served as a B-17 Flying Fortress bombardier in the Eighth Air Force. At age 44 he was working as a farmer in California when he decided to join the Peace Corps. After serving as a volunteer for four years, he remained in Ecuador. He died in 1991 of cholera.

Thomsen published four books, three of them memoirs. Living Poor: A Peace Corps Chronicle, was published in 1969 and is ranked as one of the best Peace Corps memoirs ever written. The Farm on the River of Emeralds is a memoir of his years in Ecuador. My Two Wars (published posthumously) looks at both his "tempestuous" relationship with his father and his experiences as a World War II bombardier.

The Saddest Pleasure: A Journey on Two Rivers won a 1991 Governor’s Writers Award (now the Washington State Book Awards). In his introduction to The Best Travel Writing, 2005, author Tom Miller writes that The Saddest Pleasure “embodies some of the very finest elements of travel literature: constant doubt, a meddlesome nature, and a disregard for nationalism.”

Miller calls Thomsen “one of the great American expatriate writers of the twentieth century” and describes him as “A soft-hearted cuss, a man of almost insufferable integrity, a lousy farmer and a terrific writer.” He describes Thomsen’s style as one that “pledged allegiance to nothing except his station as an expatriate. And as an expat he was free to judge us all, an undertaking he finessed with acute observations, self-deprecation, and a flavorful frame of reference that ranged from a Tchaikovsky symphony to a Sealy Posturpedic mattress.”

A fifth Thomsen book, Bad News From the Black Coast, is still unpublished.

Thomsen's worldview is reflected in a statement he once made: "Living Poor is like being sentenced to exist in a stormy sea in a battered canoe, requiring all your strength simply to keep afloat; there is never any question of reaching a destination. True poverty is a state of perpetual crisis, and one wave just a little bigger or coming from an unexpected direction can and usually does wreck things."

Living Poor: A Peace Corps Chronicle (1969)The Farm on the River of Emeralds (1978)The Saddest Pleasure: A Journey on Two Rivers (1990)My Two Wars (1996) (published posthumously)