“Thinking is the hardest work there is, which is probably the reason why so few engage in it” is a slightly changed quotation from American industrialist Henry Ford (1863-1947). “Thinking is the hardest thing that’s done in the world, and the most commonly shirked, therefore” was cited in print in an 1884 newspaper. The Evening World (New York, NY), on August 28, 1889, ended a story about Thomas Edison with the words, “Real thinking is the hardest work in the world, and that is the reason, probably, why there is so little of it done.”
 
Henry Ford wrote at least two versions of the saying. “Thinking is the hardest work any one can do, which is probably the reason we have so few thinkers” was cited in print in 1921. “Thinking is the hardest work there is, which is the probable reason why so few engage in it” was cited in 1928.
 
   
Wikipedia: Henry Ford
Henry Ford (July 30, 1863 – April 7, 1947) was an American industrialist, the founder of the Ford Motor Company, and sponsor of the development of the assembly line technique of mass production. Although Ford did not invent the automobile or the assembly line, he developed and manufactured the first automobile that many middle class Americans could afford. In doing so, Ford converted the automobile from an expensive curiosity into a practical conveyance that would profoundly impact the landscape of the twentieth century. His introduction of the Model T automobile revolutionized transportation and American industry. As owner of the Ford Motor Company, he became one of the richest and best-known people in the world. He is credited with “Fordism”: mass production of inexpensive goods coupled with high wages for workers. Ford had a global vision, with consumerism as the key to peace. His intense commitment to systematically lowering costs resulted in many technical and business innovations, including a franchise system that put dealerships throughout most of North America and in major cities on six continents. Ford left most of his vast wealth to the Ford Foundation and arranged for his family to control the company permanently.
 
Chronicling America
24 February 1884, The Daily Morning Astorian (Astoria, OR), “The Man Who Knew Men: A Character Sketch,” pg. 1, col. 1:
“Thinking is the hardest thing that’s done in the world, and the most commonly shirked, therefore.”
 
Chronicling America
28 August 1889, The Evening World (New York, NY), Extra, pg. 2, col. 3:
THE INVENTOR’S GENIUS.
A recent interview with EDISON, in which “the wizard” described his method of developing and perfecting an invention, goes far towards justifying the definition which holds that “Genius is only great patience.”
(...)
Real thinking is the hardest work in the world, and that is the reason, probably, why there is so little of it done.
 
Chronicling America
21 May 1921, Dearborn (MI) Independent, “Mr. Ford’s Page,” pg. 5, col. 1:
An educated man is not one whose memory is trained to carry a few dates in history, but one whose mind can accomplish things. A man who cannot think is not an educated man, however many college degreeshe may have acquired. Thinking is the hardest work any one can do, which is probably the reason we have so few thinkers.
 
Google Books
My Life and Work
By Henry Ford, in collaboration with Samuel Crowther
Garden City, NY: Garden City Publishing Company, Inc.
1922
Pg. 247:
Thinking is the hardest work any one can do — which is probably the reason why we have so few thinkers.
 
13 April 1928, San Francisco (CA) Chronicle, pg. 24, col. 6:
Thinking Is Hardest Work,
Therefore Few Engage in It
Henry Ford in The Forum
(...)
Thinking is the hardest work there is, which is the probable reason why so few engage in it.
 
Google Books
How to Develop Your Executive Ability
By Daniel Starch
New York, NY: Harper & Brothers
1943
Pg. 70:
“Thinking is the hardest work there is,” said Henry Ford, “which is the probable reason why so few engage in it.”
 
Google Books
The Complete Thinker:
The Marvelous Mind of G. K. Chesterton
By Dale Ahlquist
San Francisco, CA: Ignatius Press
2012
Pg. 24:
Thinking is a skill. Like any skill, it can be taught. It is a simple skill, but it is still hard work.  In fact, Chesterton says that thinking is the hardest work in the world. And hard work, he says, is repugnant to our nature.
 
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8:19 AM - 3 Nov 2014