Biblical Times: The benefits of humor makes an appearance in the Good Book itself: The Book Of Proverbs 17:22 states ‘A cheerful heart does good like a medicine: but a broken spirit makes one sick’.

14th Century: French surgeon Henri de Mondeville used humor therapy to aid recovery from surgery. He wrote "Let the surgeon take care to regulate the whole regimen of the patient's life for joy and happiness, allowing his relatives and special friends to cheer him and by having someone tell him jokes."

16th Century: Robert Burton, an English parson and scholar used humor as a cure for melancholy.

16th Century: Martin Luther used a form of humor therapy as part of his pastoral counseling of depressed people. He advised them not to isolate themselves but to surround themselves with friends who could joke and make them laugh.

17th Century: Herbert Spencer, sociologist used humor as a way to release excess tension.

18th Century: Immanuel Kant, Germany philosopher used humor to restore equilibrium.

18th Century: English physician William Battie used humor in treatment of the sick.

20th Century: Modern humor therapy dates from the 1930s, when clowns were brought into U.S. hospitals to cheer up children hospitalized with polio.

20th Century - 1972: The Gesundheit Institute is founded by U.S. Doctor Hunter ‘Patch’ Adams: this is a home-based free hospital ‘to bring fun, friendship, and the joy of service back into health care’.

20th Century - 1979: Norman Cousins publishes his book ‘Anatomy of an Illness’ based on his own experiences: he had been suffering from ankylosing spondylitis and decided to use his own brand of ‘humor therapy’ by watching episodes of the television show ‘Candid Camera’ and Marx Brothers films. He claimed that ten minutes of laughter could give him two hours of pain relief.

20th Century - 1998: Renewed interest in the uses of humor as therapy thanks to the release of the film ‘Patch Adams’ starring Robin Williams and based on the real Hunter ‘Patch’ Adams.
From: http://www.freewebs.com/laughtertherapy/humourtherapy.htm
 
From Noreen:
The above list is nowhere near comprehensive, but you get the idea!
Here we are in the 21^st century, and laughter wellness has become a worldwide phenomenon. The Association for Applied & Therapeutic Humor gathers research, provides education and training to help validate the importance of humor and laughter. Laughter Yoga, originally started by Dr. Madan Kataria, introduced the idea that laughing as an exercise is as beneficial to the brain and body as spontaneous laughter. Versions of his “laughter club” idea can be found in many countries around the world, including through thousands of practitioners in the United States. (full disclosure – that includes me!) Research goes on — it has been discovered that laughter affects the brain in the same way as meditation, laughter can help in lowering blood pressure and glucose, and anthropologists are considering that laughter may have been a communications tool for humans long before language developed.