MADAME X
Sargent learned the craft of painting in Paris, where he lived for some time, but what prompted his move to London? He had exhibited paintings at the Paris Salon for quite a few years to the raving applause of all critics. He was famed as one the best portrait painters in Paris and had trained in the École des Beaux-Arts and also under Carolus-Duran from whom he might have been influenced to do portraiture, as he was more inclined to do landscapes when he started. He was also influenced by the works of Velázquez, although he might have gotten that from Carolus-Duran as well, as Duran studied Velázquez fervently.
Of all the exhibit artworks in the Paris Salon by Sargent the most critized and some would say shocking at that time was the portrait of Madame X (originally called Portrait de Mme, maybe to hide the subjects identity, but was unsuccessful in this as her own mother requested the painting to be removed from the Salon). Her pose and her pale skin as well her black dress with one strap hanging down from the shoulder (later fixed by Sargent to show as we now see it in the Metropolitan Museum of Art with the strap straight on her shoulder and at the same time changed the name of the painting to Portrait of Madame X, to give it a more mysterious tone to the painting.), the sexuality of the portrait itself was alarming to the France society which received it with shock. Unfortunately it created a scandal and Madame Gautreau (the lady posing for the portrait) was humiliated by the affair and Sargent would move to London. Interestingly enough the portrait was pursued by Sargent himself, he asked a mutual acquaintance to set up a meeting to discuss a portrait painting, not customary as most or all portraits of the time were commissions.
Today it's known as Sargent's masterpiece, as he himself said in a letter to the director of the Met when he sold it; "I suppose it is the best thing I have ever done."