It is widely attributed that Sargent stopped taking portrait commissions in 1907, though I had always suspected that upon his mother's death and inheriting her money, he was able to live without portrait commissions and his decision became easier. 

That in itself is an over simplification. By 1906 Sargent was making a small fortune from his portraits and he possibly could have gone without any more portrait commissions irregardless of his mother's money (that might be a fun question to explore).  

What we clearly know is Sargent was always looking for a new challenges and the literature is full of information that tells how Sargent didn't care much about making a lot of money. By 1903 when he exhibits 30 works at the Carfax Gallery in London, England, comprised of watercolors and sketches, you can see Sargent thinking about moving in a different direction -- as if he was testing the waters. 

But his mother, I believe, loved that he did portraits. She had been quite the social climber when John was young (though never to rival the professional social climbers in New York, Boston, and London) throwing parties to introduce him to influential people when they traveled and I could quite imagine that she was very proud of her son painting the richest and the most powerful of the time. 

But Sargent hated society types and anyone who put on heirs. He felt the most at home with his Spanish Gypsies, his bohemian art friends, writers, musicians and actors as he did with his gentry. 

After his mother's death, every autumn Sargent would travel to Europe with his sister Emily and one if not more of a small circle of friends who included: the de Glehns, Miss Eliza Wedgwood, Misses Barnard or Sargent's sister Vilot and her children. And they would travel to Val d'aosta, Italy, Corfu, Majorca or Spain. It is said that these trips comprised the happiest times of his life (Charteris P.170) 

So although the bulk of the literature establishes the time of Sargent's giving up portraits as 1907, I think we can see a restlessness as early as 1903 and after the death of his mother in 1906, the desire, or obligation to please her is no longer binding. In a sense, Sargent is freed and it is that critical date, I believe, we should pin the date of Sargent's decision to stop painting portraits (not to imply that he didn't love his mother, for he very much did). 

See the years in review 1905-06 
 

From: Bert  
10/25/99  

Regarding JSS retiring from portraiture. I find this a charming account. From pages 227 - 228 of the Olson JSS biography 
 

By 1907 he had had enough. At the age of fifty-one he shut his ledgers and hung a "closed" sign on the door. It had been coming for a long time:  "Painting a portrait would be quite amusing if one were not forced to talk while working," he said to Jacques-Emil Blanche. "What a nuisance having to entertain the sitter and to look happy when one feels wretched." He could not go on with the lie any more. "I have vowed a vow",  he wrote to Mrs Curtis, "not to do any more portraits. . . it is to me positive bliss to think that I shall soon be a free man."  "No more paughtraits," he repeated to her son, using his individual spelling of the dreaded word, ". . . I abhor and abjure them and hope never to do another especially of the Upper Classes." To Lady Radnor he was as emphatic: "Ask me to paint your gates, your fences, your barns, which I should gladly do, but not the human face." His disenchantment reached its most cogent form with the terse declaration, "No more mugs!", and later settled down in the definition he inscribed on the flyleaf of the French edition of reproductions of his work: "A portrait is a picture in which there is just a tiny little something not quite right about the mouth."  

And thus resolved "to devote my life to something more satisfying" he submitted four portraits to the Academy in 1908: an ex-prime minister (Balfour), a royal duke (HRH The Duke of Connaught) and his Duchess, and one other (Mrs Huth Jackson); the following year two more — Mrs William Waldorf Astor* and the Earl of Wemyss — and a landscape. In 1910 he sent only landscapes. The great wheels of the portrait works had come to a halt.

Thank you Bert. That was wonderful. But did Sargent ever REALLY come to a halt in portraits?  

The answer is no. Even the last day of his life he was found doing one of his countless charcoal sketch-portraits which he called "mugs" of Princess Mary. And you find him cornered into doing full oil portraits such as Abbott Lawrence Lowell in 1923 and his last oil portrait four months before his death in 1925 of Grace Elvina.  

Part of the problem in identifying when he stopped portraits is that he never really did -- not completely. Though he certainly wanted to and he grew to even hate the "mugs" he did. Inherently, Sargent had a good heart and as much as he wanted to, it was hard for him to say no to his friends. The excerpt you sent shows a great example of this:  

"Ask me to paint your gates, your fences, your barns, which I should gladly do, but not the human face."  

He's pleading with her. 
 

From: Wonsug Jung 
1/16/00 

While reading Mount's book, I find this paragraph about Sargent's mother's death. 

Mrs. Sargent's estate was small, only thirty thousand dollars, on the income from which both she and Emily had been living in London. It was divided equally between John and his two sisters. But the obvious intention of his mother had been to care for Emily, her will touchingly stating that her son "through his great talent" was able to provide for herself and Violet's husband could look after her, while Emily would have nothing but this inheritance. She had already given Emily her personal possessions for that reason and there were really adequate means for Emily to live on, almost indefinitely, at her modest rate. It must at this time have occurred to him, suddenly, how great his own earnings were. Compared with his parents, he was fabulously rich.

Wow, what a find! I just had assumed (which always gets me into trouble) that since JSS's parents hadn't been working his entire life (I knew they weren't considered wealthy by aristocratic standards) I assumed the estate of his mothers would have been a lot more than that (of course $30,000 U.S. would have been a lot more in 1907 than it is today). Still, it puts to bed my assumption that any inheritance added to his decision to quit portraits.