Mattel released this in 1976, it was inspired by some of the arcade racing games in the 70s. It used a series of red LEDs to simulate cars on a track. the player controls a car on a three-lane track and moves between them with the switch with a car icon and could change gears using the top left switch.
There were many other one off handheld games produced afterwards in a similar form factor such as: Football, Baseball, Missile Attack, and Ski Slalom.
A Brief History of Video Game Handhelds Part 1: 1975-1990
Mattel released this in 1976, it was inspired by some of the arcade racing games in the 70s. It used a series of red LEDs to simulate cars on a track. the player controls a car on a three-lane track and moves between them with the switch with a car icon and could change gears using the top left switch.
There were many other one off handheld games produced afterwards in a similar form factor such as: Football, Baseball, Missile Attack, and Ski Slalom.

There were many single game systems produced throughout the 70s and 80s of various quality. Usually licensed from popular arcade titles.
Many companies produced these such as Coleco and Milton Bradley.

Merlin The Electronic Wizard was created by former NASA engineer Bob Doyle alongside his wife and brother in-law for Parker Brothers in 1978.
Considered the first commercially successful handheld game system with over 5 million units sold.
The "play area" had 11 buttons, each with its own LED, the buttons can either be solid or flash. It featured six built in games: Tic tac toe, Music machine, Echo, Blackjack, Magic square, and Mindbender.

In 1979 Milton Bradley introduced the Microvision, the world's first cartridge based handheld. Unlike most later consoles, the Microvision did not contain an onboard processor (CPU). Instead, each game included its own processor contained within the removable cartridge. This meant that the console itself effectively consisted of the buttons and LCD panel.
The produced 12 cartridges before it was discontinued in 1981. While it was initially very popular... It was unfortunately prone to many problems such as ESD damage (static), screen rot (basically the crystal would leak out of the panel), and buttons breaking easily. It also lacked support from many of the big game makers at the time.

In 1980 Gunpei Yokoi pitched the idea of the Game & Watch to Nintendo president Hiroshi Yamauchi after watching a bored businessman play with his calculator, just tapping the buttons randomly, on their daily train commute to work. He thought that a watch that doubled as a game system would be a great time killer. A week later he was approved to start designing the first system in the series.
They would go on to create 62 total Game & Watch systems. Utilizing segmented LCDs, pre-printed with an overlay, limiting each model to only play one game. Many games had a button to toggle between Game A and Game B. Game B was usually a sped up, harder version of the Game A. The series would go on to sell 14 million units between 1980 and 1991.
Probably the most important one they created was a port of Nintendo's insane smash hit arcade game: Donkey Kong. Yokoi created a cross-shaped mechanism to control movement with a ball pivot in the center beneath it to help keep the buttons from pressing the wrong contacts. This was the origin of the D-pad as we know it today. Nintendo would go on to utilize this design on controllers for the NES, SNES, etc.

The Game Boy was Nintendo's second foray into handheld gaming. Following their success with their smash hit home console the Nintendo Entertainment System, this cartridge based handheld further cemented Nintendo's dominance in the gaming market.
Released in 1989 in Japan and the USA for the price of $90, it would go on to sell over 64 million units. Created by Chief Engineer Gunpei Yokoi and the R&D1 team, the system featured an 8bit processor, a monochrome screen that could display 4 shades of green, and a mono speaker. It had a link cable port on the side to allow for multi-player games.
It had a very rugged design and is extremely durable, with one famous unit surviving a bombing during the Gulf War. It can still be seen on display at the New York Nintendo Store still working perfectly, even 30 years later.
Over 1000 games were produced for the Game Boy with some notable titles including: Tetris, Super Mario Land, The Legend of Zelda: Link's Awakening, Rampart, Pokemon Red/Blue/Yellow, and many, many more...
It is one of the most recognizable devices from the 1990s, becoming a cultural icon in the years following its release. Several redesigns were produced during its lifetime, including the Game Boy Pocket in 1996 (which was about half the size of the original) and the Game Boy Light (which featured a backlit screen) in 1998 (Japan only).
It could run for over 30 hours on 4 AA batteries.

Released 2 months after the Game Boy in 1989 The Atari Lynx was originally developed by Epyx who unfortunately went bankrupt during the development process. Priced at $180 USD at launch, it would go on to sell 2 million units.
It featured a full-color backlit LCD screen and could display 16-bit graphics. The Lynx was cited as the first gaming console with hardware support for zooming and distortion of sprites, a feature later utilized to great effect by the SNES and Genesis.
Atari published a total of 73 games for the Lynx before it was discontinued in 1995 with notable titles including Ms. Pac-Man, Ninja Gaiden III, Paperboy, and Rampage.
In order to load games from the cartridge it first had to copy the entire ROM to the system's RAM. This took some time to do and left very little for anything else that needed to utilize it, which sometimes lead to performance issues.
Atari shifted focus to a new home system, the Jaguar, which led to fewer games developed for the Lynx overall.
It could run for 4-5 hours on six AA batteries.

In 1990 NEC released the TurboExpress for the price of $250. The TurboExpress was technically advanced for the time, able to play all the TurboGrafx 16's games. It had a backlit color LCD screen and a TV tuner.
Yeah, you could watch TV on this thing.
Unfortunately due to a problem with faulty capacitors (an industry-wide issue in the early 1990s) sound failure is a frequent problem with the TurboExpress.
The screen was another source for problems. The LCD technology used was still fairly new and the rate of pixel failure was very high. It sold 1.5 million units, making it the worst selling of the original Game Boy era of handhelds.
It ran for 3 hours on 6 AA batteries.

Following the success of the Game Boy Sega released its own cartridge based handheld, the Game Gear, in Japan in 1990 and then in the US in 1991 for the price of $150.
It shared much of its hardware with the Sega Master System and can even play Master System games with an adapter. Its major advantage over the Game Boy was its full-color backlit display as well as its more ergonomic form factor. It would go on to sell over 10 million units.
366 games were created for the Game Gear with notable titles including: Battletoads, multiple Sonic the Hedgehog games, Bust-A-Move, and Wonder Boy: The Dragon's Trap.
Despite its superior technology to the Game Boy it had one fatal flaw. It only ran for 3-4 hours on six AA batteries, making it very impractical for actual portable play.

These cheap ass things plagued the 90s by using many of the same components that were developed in the 80s. Tiger had an incredible licensing team and was able to remain profitable due to the low cost of these systems which usually around $20.
