There was a time when it just wasn’t a party without a Jell-O mold.
Just like fashion, the food industry experiences plenty of trends. These days certain ingredients become trendy, as do different types of cuisines, specific dishes, as well as certain cooking techniques. Some big food trends of recent times include cupcakes, bacon, sous vide cooking, tapas, fusion tacos, and food trucks. Many different trends come in and out of popularity in American culture and some become classics that continue to live on, such as the hamburger.
Perhaps you remember a major food trend of the 1950’s and 60’s – the Jell-O mold. This jiggly, colorful dish was once a staple of dinner parties and potlucks and naturally lends itself to seemingly endless variations. While the use of gelatin, derived from animal bones, in culinary applications has been around for hundreds of years, the Jell-O mold itself is a product of modern times. It was the French who first started using gelatin in their cuisine to create intricate molded dishes suitable for elaborate banquets for kings and queens.
In 1894 Charles Knox developed the first pre-granulated gelatin available for home use. Knox hired salesmen to go door to door teaching women how to use the product to make many different molds and desserts. The Jell-O brand itself was created in 1895 by Pearl B. Wait and his wife, May, who experimented with adding fruit and sugar to powdered gelatin. They were not very successful so they sold the company to their neighbor Orator Francis Woodward. It took him several years to create much of a market for the product and in 1902 he launched an advertising campaign dubbing Jell-O “America’s most favorite dessert.” Jell-O took off and over the years added different flavors and other products including pudding, custard and flan. The popularity of Jell-O molds and congealed “salads” prompted the company in the 1960’s to create a line of savory flavors including celery and Italian. Popular recipes from this time included using chopped vegetables and even pasta in the molds. In 1964 the now iconic advertising slogan “There’s always room for Jell-O” was created to brand the dish as a good choice for a light dessert.
Jell-O molds may not be the star of every dinner party anymore, but there has been a resurgence in popularity in recent times, including a Jell-O mold competition in New York City. There is a kind of nostalgia associated with Jell-O molds and with the wide availability of high-quality ingredients it is easy to elevate the humble mold to new gourmet levels.

A classic Jell-O mold salad.
Source
Sources:
http://whatscookingamerica.net/History/Jell-0-history.htm
http://www.fourpoundsflour.com/origin-of-a-dish-the-jell-o-mold/