Although I don't know who's the first muckraker, here's a block of info.
"The term muckraker most associated with a group of American investigative reporters, novelists and critics from the late 1800s to early 1900s, who investigated and exposed societal issues such as conditions in slums and prisons, factories, insane asylums (as they were called at the time), sweatshops, mines, child labor and unsanitary conditions in food processing plants. Muckrakers often wrote about impoverished people and took aim at the established institutions of society, sometimes in a sensationalist and tabloid manner. (See History of American newspapers for Muckrakers in the daily press). Muckrakers were often accused of being socialists or communists."
