The ice cream soda was invented by a soda fountain manufacturer named Robert McKay Green from Philadelphia, PA. Cream soda was popular at the time, made with carbonated water, cream and syrup. The most popular was vanilla.
Green is said to have created it while tending a concessionaire at the 1874 Semicentennial Franklin Institute Exposition in Philadelphia. One story says he ran out of cream and used vanilla ice cream from a neighboring booth instead, but his own account records that it was more of a deliberate invention created to attract customers away from a much larger competitor. Sales soared and his treat made a big splash.
Two years later, fountain operators at the 1876 Centennial Exposition in Philadelphia (the first official World's Fair in the United States, commemorating the signing of the Declaration of Independence) gave many tourists their first taste of the beverage. Green’s ice cream soda became a hit around the world.
Click here for the recipe: http://www.csindy.com/IndyBlog/archives/2015/07/04/how-to-make-a-coffee-ice-cream-soda