All Pratt's first appearance was in All-American Comics #19 (October 1940), an anthology title that starred the Golden Age Green Lantern Alan Scott (who had debuted in issue #16).
In that story, the college student called "Atom Al" because of his short size is constantly ridiculed and pushed around by other (taller) students. When he and the (taller) girl he's interested in are then robbed (and Al does nothing to try and stop the thief), she calls poor Al a "spineless little amoeba" and says she never wants to see him again!
Al then helps a man who begs for a dime (hey, it's 1940!) by instead taking him to buy a meal. He learns the man was once a successful fight trainer named Joe Morgan, so Al convinces Joe to train him so he can stand up for himself when others try to push him around. The training results in, well, Al being much stronger than anybody would have imagined!
On the last page, Al saves the girl who previously called him "spineless" by taking out a bunch of crooks who had kidnapped her! He hadn't adopted his costume yet, but the girl (Mary) never saw who saved her (she was blindfolded the entire time). Al did leave behind a card before he left that simply said