The image of the old, good-natured St. Nicholas, bishop of Myra, in Russia, has been transformed, over time, into the well-known figure of Santa Claus. Although the basic elements had been well-defined for a long time (beard, clothes, etc) only a Coca-Cola ad campaign would turn it into the famous image we know and love today. Its author was Haddon Sundblom.
It was 1931 when an American illustrator was asked by Coca-Cola to draw up that year's Christmas campaign. Note that the company had been using the image of Santa Claus to promote its drink since 1920. One of the illustrators that preceded Sundblom was none other than Norman Rockwell, who in spite of his magnificent drawings hadn't been able to attain much advertising success.
Sundblom institutionalized the image of Father Christmas by ignoring the similarities with an elf of previous versions, but rather by giving him an unmistakably human look in his physiology and, most of all, face. It has been said that he used a human model - a friend of his, Lou Prentice - and later he, himself, served as a model. Furthermore, he was stubborn enough, or intuitive enough, to draw the same image in every campaign, until 1964. This persistence paid off and resulted in the consolidation of this character as we know it today - as the big-bellied old man with a long white beard, rosy cheeks and youthful smile.
The origin of the Santa Claus cult is explained here.