Born in Cardiff, Wales March 14, 1922, Fletcher was educated in England and served six years in the Royal Marines during World War II. After spending time farming in Kenya, surveying in Zimbabwe, and as a prospector in western Canada, he moved to the United States in 1956. Two years later, he walked the length of the state of California, a journey that was the basis for his first book The Thousand Mile Summer.

Fletcher published a total of 10 books between 1964 and 2001, which included 4 editions of the The Complete Walker. His first book was The Thousand Mile Summer (1964) recounting his 1958 hike along the entire eastern edge of California. His second book was The Man Who Walked Through Time (1968), in which Fletcher was the first person to walk a continuous route through Grand Canyon National Park in 1963. The book covered technique, the journey itself, and reflections which included the concept of, after weeks, of achieving a state of mentally "merging" with the place that one is visiting.

In 1968, Fletcher published the first edition of The Complete Walker, his most popular work including three new editions with the last in 2001, in total selling over 500,000 copies. Fletcher's book is distinguished by its encyclopedic treatment of the technique and equipment of wilderness travel, as well as by what critics and readers have praised as its rousing humor and elegant, vigorous prose. While certainly comprehensive in its discussion of all aspects of wilderness travel, it also devotes a generous amount of space to Fletcher's self-confessed idiosyncrasies, ranging from his affection for walking staffs and corduroy shorts to his loathing of wilderness trail guidebooks.

In the early 1970's Fletcher returned to Kenya's Serengeti Plain, and Great Rift Valley for a year, an experience he recounted in "The Winds of Mara", published in 1973.

In 1981 he published The Man From the Cave, which tells how, after finding a trunk and a few belongings abandoned by someone in a desert cave in Nevada, he spent years piecing together the life story of "Trunkman". As he pieced together the mystery of the man's life, Fletcher saw in it a discovery and reflection of himself, "We both valued solitude and silence and square, smoothed-off granite boulders."

In 1989, Fletcher hiked and rafted the entire length of the Green/Colorado River from its source in the Wind River Range of Wyoming to the Gulf of California. He was evidently the first person ever to do this. He told the story of the journey in River (1997), adding to his customary description of the places through which he travelled a metaphor, comparing the course of the river and its emptying into the sea, to life itself and his own life in particular.

The last edition of The Complete Walker, The Complete Walker IV (2001), was written with Chip Rawlins.

In his later years, Fletcher became a prolific writer on environmental issues. He was publicity-shy, rarely responding to letters or interview requests, although always willing to incorporate reader feedback into revised editions of The Complete Walker.

In 2001, at the age of seventy-nine, Fletcher was struck and seriously injured by an SUV while walking to a town meeting near his home in Monterey County, California. His survival was attributed to his excellent physical condition. Within a year of the accident, he was back on his feet and walking daily.

According to published obituaries, Fletcher died on June 12, 2007 in Monterey, California, as a result of complications from a head injury sustained from being hit by the car six years earlier.

Annette McGivney, editor of Backpacker Magazine, has written that "Colin was sort of the founding father of modern backpacking, the first person to write about going out for an extended period and being self-sufficient." Because many people started following Fletcher's advice in The Complete Walker, according to McGivney, "the book could be credited with starting the backpacking industry," including equipment makers and periodicals like Backpacker Magazine. Through his writings Fletcher "inspired a generation of young Americans to take up backpacking as means of filling a spiritual void," and to escape from the confusion of Vietnam-era America:

"After Vietnam, I was trying to figure out what to do with my life. So many of my friends had died from bullet holes," recalls Backpacker Magazine contributing editor Buck Tilton. "I read The Man Who Walked Through Time, and it was the only thing that made sense to me. Fletcher's words gave meaning to backpacking. I loaded my pack exactly the way Fletcher did and carried a walking stick like his. He was my hero."

• God is light, we are told, and Hell is outer darkness. But look at a desert mountain stripped bare by the sun, and you learn only geography. Watch darkness claim it, and for a moment you may grasp why God had to create Satan...or man to create both. ... The Thousand Mile Summer (1964)

• Dedicated urbanites "know" beyond shadow of doubt - because doubt never raises its disturbing head - that civilization is the real world: you only "escape" to wilderness. When you're out and away and immmersed, you "know" the obverse: the wilderness world is real, the human world a superimposed facade... The controversy is, of course, spurious. Neither view can stand alone. Both worlds are real. But the wilderness world is certainly older and will almost certainly last longer. Besides, the second view seems far healthier for a human to embrace. ... River (1997)