1979: Milton-Bradley Microvision game console
Ask most people to name the first cartridge-based handheld game machine, and you'll likely get the wrong answer. It wasn't 1989's original Game Boy from Nintendo; it was Milton Bradley's $50 Microvision, introduced for Christmas 1979. Advancing beyond single-game devices like Mattel's 1977 Auto Race, Microvision had swappable cartridges, making it the first true handheld gaming system.
Thanks to a quirk of design, the cartridges contained the CPU as well as a game; the console housed the display, input buttons and 9-volt batteries. It took some imagination to see starships in the chunky 16 x 16 LCD pixels while playing Phaser Strike on the postage-stamp screen, but it worked for the Pong generation.