There is a great disagreement in New Thought over who is really the founder of New Thought. This is because the definition of New Thought was created after the movement began. Thus in truth, the movement has its roots in the evolution of thought and thought is as old as life itself.
Nevertheless, there are many within the New Thought movement who believe the essence of New Thought is rooted in the interpretation of Quimby's teachings, and would like to credit Quimby as the intellectual father of New Thought.
This is a much disputed point among the eggheads of minutia who lurk in the conferences and galleries of the dusty divans of New Thought's surviving early institutions. But you can judge for yourself by reading his work:
Phineas Parkhurst Quimby, who was known as "Park," was born on February 16, 1802, in Lebanon, New Hampshire. He was apprenticed as a clockmaker and had little traditional education. Several important elements of his life led to the development of his ideas of mental healing.
The first important milestone was when Quimby developed tuberculosis but became disillusioned with the method of treatment prescribed by his physician and gave up hope of recovery. A friend suggested that he take up a physical outdoor activity such as horseback riding to improve his condition. While his severe physical ailments prevented him from trying this suggestion, Quimby tried the next best thing and embarked on carriage trips. This course of action produced remarkable results and his recovery prompted much thought on the matter. However, he did not pursue this further until several years later.
In 1838, Quimby began studying Mesmerism after attending a lecture by Doctor Collyer and soon began further experimentation with the help of Lucius Burkmar, who could fall into a trance and diagnose illnesses. Quimby again saw the mental and placebo effect of the mind over the body when medicines prescribed by Burkmar, with no physical value, cured patients of diseases. From the conclusions of these studies, Phineas Quimby developed theories of mentally aided healing and opened an office in Portland, Maine in 1859. Among the students and patients who joined his studies and helped him to commit his teachings to writing were Warren Felt Evans, Annetta Seabury Dresser and Julious Dresser, the founders of New Thought as a named movement, and Mary Baker Eddy, the founder of the Christian Science movement.