In 1902, 65 years after Queen Victoria appeared on an English postage stamp, and 55 years after the first US postage stamp was issued, the 8-cent Martha Washington was the first US stamp to feature a woman.
The year before, the Post Office Department had enjoyed a successful run of the commemorative Pan-American Exposition stamps. But, at more than a decade old, all its regular-issue designs were becoming a bit dated. In an attempt to jazz up its offerings, the Postal Department commissioned new stamps for 1902, giving artists and engravers free rein to show off their intricate handiwork.
Mrs. Washington's stamp was introduced at a time when Americans began recognizing the importance of women's contributions to society. Although women had not yet earned the right to vote, the suffrage movement was a major topic of conversation. Legend has it that after much squabbling over which prominent woman should be the first to grace a US stamp, Mrs. Washington -- the first first lady -- evoked the least controversy.
She later appeared on 1, 1-1/2 and 4-cent stamps. (info from Yahoo and other sources)