Actually, Marlboros were marketed as a cigarette for women, starting in 1924. The slogan used was "Mild as May", but the smokes weren't really popular, as Philip Morris only had about one quarter of one percent of the market up until the 1950s.
The Marlboro cowboy didn't arrive on the scene (along with other "manly man" images like sailors and construction workers ... sounds like the inspiration for The Village People, if you ask me) until 1954, thirty years after Marlboro had come onto the market, and the phrase "Marlboro man" didn't attain popularity until around 1957 or 1958.
The Marlboro cowboy became the sole "Marlboro man" in 1963. In 1964, the phrase "Come to Marlboro Country" was introduced, and this is when Marlboro began attaining popularity amongst smokers, with sales increasing about 10% a year.
In 1970, Marlboro was ranked third in world market share (behind Winston and Pall Mall), becoming the best selling smoke in 1972, a place it lost to Winston briefly, regaining the top spot in 1975. Philip Morris, however, didn't become the #1 cigarette manufacturer in the USA until 1983.
Marlboro remains the best selling cigarette brand in the world, selling nearly 7 billion packs a year in 1990, far ahead of Winston, selling a paltry 2 billion packs a year.
Y'know. I should quit smoking.
Most of the information found here
was adapted from "The History of Tobacco",
located at History Net, www.historian.org