Thomas Nast is the first well-known and
influential illustrator to draw Santa Claus
annually over a long period of time (23 years,
from 1863-1886) for a major magazine
("Harper’s Illustrated Weekly").
Four brothers, James, John, Wesley and Fletcher
Harper, founded the Harper & Brothers
publishing company in 1825. They began
publishing "Harper’s Monthly" in 1850, and in
1857 they began publishing "Harper’s Weekly"
in New York. By 1860 the circulation of the
"Weekly" had reached 200,000. It was the most
widely read journal in the United States
throughout the period of the Civil War.
Albert Boime, in his essay “Thomas Nast and
French Art” for the "American Art Journal"
(1972) 4 # 1 pp. 43-65, argued that “as a
political cartoonist, Thomas Nast wielded more
influence than any other artist of the 19th
century. He not only enthralled a vast audience
with boldness and wit, but swayed it time and again to his personal position on the strength of his visual imagination. Both Lincoln and Grant acknowledged his effectiveness in their behalf, and as a crusading civil reformer he helped destroy the corrupt Tweed Ring that swindled New York City of millions of dollars. Indeed, his impact on American public life was formidable enough to profoundly affect the outcome of every presidential election during the period 1864 to 1884.” https://www.yumpu.com/en/document/view/11489343/thomas-nast-and-french-art-professor-albert-boime
Put the two together (man and magazine) and it’s not hard to understand how the Nast Santa became the defining image for years after.
Thomas Nast was born in Landau, Germany, on Sept. 27, 1840. When he was six years old, He arrived in New York City with his mother, when he was six, and his father arrived four years later. After art studies, Nast became an illustrator for "Frank Leslie's Illustrated Newspaper" at the age of 15. By the time he was 19, he was working at "Harper's Weekly" and he later traveled to Europe on assignment for other publications. Soon he was a famous political cartoonist. http://german.about.com/library/blnikolaus3.htm
While his first depiction of Santa Claus appeared in 1862 as part of "Christmas Eve", the figure is too small to allow for detail.
One year later Nast's “Santa Claus in Camp” was on the cover of the January 3, 1863, issue of "Harper's Weekly", and he continued drawing illustrations of Santa Claus for "Harper’s" until 1886. He contributed 33 Christmas drawings to “Harper’s Weekly” during that time and all but one referenced or showed Santa Claus. https://www.nytimes.com/learning/general/onthisday/harp/1225.html#explanation
“Before Nast, different regions, ethnic groups, and artists in the United States presented Santa Claus in various ways. A sketch in "Harper’s Weekly" from 1858 shows a beardless Santa whose sleigh is pulled by a turkey. Nast was instrumental in standardizing and nationalizing the image of a jolly, kind, and portly Santa in a red, fur-trimmed suit delivering toys from his North Pole workshop. This was accomplished through his work in the pages of "Harper’s Weekly", his contributions to other publications, and by Christmas-card merchants in the 1870s and 1880s who relied heavily upon his portraiture.” https://www.nytimes.com/learning/general/onthisday/harp/1225.html#explanation
Santa Claus In Camp, by Thomas Nast: http://www.complex.com/art-design/2013/12/christmas-art/santa-claus-in-camp