[2][5] In October 1901, the Atlantic Monthly published an article by Sullivan, "The Ills of Pennsylvania", on corruption among local and state officials who were reportedly accepting bribes.

Sullivan alleged that Pennsylvania was "politically the most corrupt state in the union" and Philadelphia "the most evil large city in America".

After writing for the Ladies Home Journal about misleading advertising for patent medicines, he was hired in 1905 by McClure's as a staff writer.

[5][6] In 1906, along with Willa Cather, Georgine Milmine, Will Irwin, and Burton J. Hendrick, Sullivan became part of the McClure's team that produced a series of 14 investigative articles on Mary Baker Eddy, founder of the Christian Science church.

[2][6] When Sullivan joined the New York Evening Post in 1919 as its Washington correspondent, the newspaper's president, Edwin F. Gay, wrote: "His ability, his vision, his knowledge of human reactions and twenty years of Political study are coupled with unquestionable sincerity, plus 100 percent of rugged Americanism.

Dan Rather, who edited the material to produce one volume, wrote in 1996 that "no series of nonfiction books, all on the same general subject by the same author over such a compact space of writing time, ever captured the country so completely, sold so well, was so widely read and acclaimed, and had such a lasting, growing reputation for excellence".

[18][19] During a press briefing, Roosevelt said Sullivan was arguing that Shea had the "absolute freedom, as an American citizen, to starve to death when she got to be sixty-five if she wanted to".

[21] The couple had two daughters, Narcissa and Sydney, and a son, Mark Jr.[22] Sullivan became the owner of his parents' farm in London Grove and continued to regard it as his home.