Who started the slogan “The customer is always right”? Was it department store founder Marshall Field (1834-1906) of Chicago? The first citations in 1905 seem to indicate this.
 
“The customer is always right” has also frequently been credited to department store founders John Wanamaker (1838-1922) and Harry Gordon Selfridge (1858-1947). Swiss hotelier César Ritz (1850-1918) was credited in 1908 with “The customer is never wrong” (“Le client n’a jamais tort”).
 
“Give the lady what she wants” is another Marshall Field slogan. Another popular customer saying is “If you don’t take care of your customers, someone else will.”
 
[This entry was assisted by research from the Quote Investigator.]
       
   
Wikipedia: The customer is always right
“The customer is always right” is a motto or slogan which exhorts service staff to give a high priority to customer satisfaction. It was popularised by pioneering and successful retailers such as Harry Gordon Selfridge, John Wanamaker and Marshall Field. They advocated that customer complaints should be treated seriously so that customers do not feel cheated or deceived. This attitude was novel and influential when misrepresentation was rife and caveat emptor (let the buyer beware) was a common legal maxim. Variations include “le client n’a jamais tort” (the customer is never wrong) which was the slogan of hotelier César Ritz who said, “If a diner complains about a dish or the wine, immediately remove it and replace it, no questions asked”. A variation frequently used in Germany is “der Kunde ist König” (the customer is king), while in Japan the motto “okyakusama wa kamisama desu” (お客様は神様です) meaning “the customer is a god”, is common.

However it was pointed out as early as 1914 that this view ignores that customers can be dishonest, have unrealistic expectations, and/or try to misuse a product in ways that void the guarantee.
 
27 April 1905, The Homestead (Des Moines, IA), “A Little History of the Mail Order Business,” pg. 13, col. 4:
(About Sears, Roebuck & Co. in Chicago.—ed.)
Their business and policy is the most liberal ever known. It is first and foremost, “Take care of the customer—serve the customer.” They promptly refund the money and pay all of the expenses of the transaction if any goods do not please the purchaser. Every one of their thousands of employes are instructed to satisfy the customer regardless of whether the customer is right or wrong. The customer comes first last and all the time.
 
3 September 1905, The Sunday Herald (Boston, MA), “A Modest Millionaire,” Women’s Sec.,  pg. 10, col. 2:
Marshall Field is one of the most modest and retiring millionaires and merchants in the world.
(...)
Every employe from cash boy up is taught absolute respect for and compliance with the business principles which Mr. Field practices. Broadly speaking, Mr. Field adheres to the theory that “the customer is always right.” He must be a very untrustworthy trader to whom this concession is not granted.
 
24 September 1905, Boston (MA) Sunday Globe “He Shares the Public Burden” (Marshall Field), pg. 41, col. 5:
Marshall Field is one of the most modest and retiring millionaires and merchants in the world.
(...)
Every employe from cash boy up is taught absolute respect for and compliance with the business principles which Mr. Field practices. Broadly speaking, Mr. Field adheres to the theory that “the customer is always right.” He must be a very untrustworthy trader to whom this concession is not granted.
 
11 November 1905, Corbett’s Herald (Providence, RI), “Topics of the Times,” pg. 4:
One of our most successful merchants, a man who is many times a millionaire, recently summed up his business policy in the phrase, “The customer is always right.” The merchant takes every complaint at its face value and tries to satisfy the complainant, believing it better to be imposed upon occasionally than to gain the reputation of being mean or disputatious.
   
Google Books
Tarbell’s Teachers’ Guide to the International Sunday-School Lessons for 1907
By Martha Tarbell
Indianapolis, IN: The Bobbs-Merrill Company
1906
Pg. 133:
A merchant who is many times a millionaire, recently said that he owed his prosperity to this spirit of conciliation shown by Isaac, His business policy is phrased thus, “the customer is always right”; in other words, he preferred to be imposed upon occasionally, to accept every complaint a customer might make at its face value, and adjust things to suit that customer, rather than contend the question.
 
Google Books
Piccadilly to Pall Mall:
Manners, Morals, and Man
By Ralph Nevill and Charles Edward Jerningham
London: Duckworth & Co.
1908
Pg. 94:
This maxim was “Le client n’a jamais tort,” no complaint, however frivolous, ill-grouned, or absurd, meeting with anything but civility and attention from his staff.
(César Ritz.—ed.)
     
17 September 1909, Deseret Evening News (Salt Lake City, UT), pg. 7, col. 4 ad:
“The customer is never wrong”
Was a maxim of Marshall Field. We’ve adopted that rule and in your designs with us, you will find it rigidly adhered to.
(Western Fuel Co.—ed.)
 
Google Books
16 March 1910, Printers’ Ink, pg. 43, col.1:
CARRYING OUT MARSHALL FIELD’S PRECEPT, “THE CUSTOMER IS ALWAYS RIGHT.”
Two young men who are employed in a big department store were dining together. “Well, how many times did you lose your job to-day?” asked one.
 
“I had an easy time of it to-day,” replied the other. “I was only fired six times.”
 
A friend seated at the table with them expressed surprise at this remarkable conversation.
 
“Well, you see it’s this way,” said the one who had first spoken. “Tom happens to be the stores professional fired man. There isn’t an hour goes by but some disgruntled customer comes in with a complaint about some error and demands that the person who is responsible for the error be reprimanded. That’s where Tom comes in. He is sent for and told that the mistake is due to his carelessness, and that his services are no longer required. Tom goes away, apparently crestfallen, and awaits the next summons.”—N. Y. Sun.
 
15 June 1912, Indianapolis (IN) Star, pg. 8, col. 2:
One secret of Marshall Field’s success was the motto he enforced, “Remember that the customer is always right.”

23 August 1912, Lincoln (NE) Daily News, pg. 4, col. 2:
A certain successful clothing house deliberately tells its clerks to remember that “the customer is always right.” 

11 October 1912, Trenton (NJ) Evening Times, pg. 4, col. 2: 
Our motto is: “The customer is always right.” - and we live up to our motto.
(The P. & O. Shop - ed.)

23 June 1914, Christian Science Monitor, pg. 2 (from A. A. Shryer of Detroit):
“If 50 per cent of our business men could be made to really understand how infinitely profitable it is to practice the axiom ‘the customer is always right,’ profits from advertising would increase far beyond any point of concern regarding waste.”

2 July 1915,  New York (NY) Times, pg. 3 ad:
Riker-Hegeman Drug Stores.  Drug Stores with character and a conscience.  The customer is always right.
43 New York Stores.

14 July 1918, Chicago (IL) Daily Tribune, pg. 23:
However, the motto of the store (James & Younger—ed.) was, “The customer is always right,” so, all right.

17 November 1918, Chicago (IL) Daily Tribune, pg. E10:
The business (Montgomery Ward—ed.) began in one room twenty feet square at 246 Kinzie street, Chicago. From there the firm announced by mail that they were able to ship goods direct to the customer, and that the customer should be the judge of his own satisfaction with these goods and the prices paid for them. Thus they became the pioneer mail order merchants of America, with a business based on the theory that “The customer is always right.”

26 July 1954, Chicago (IL) Daily Tribune, pg. 22:
“Give the lady what she wants,” said Marshall Field, and thus began the tradition that the customer is always right.