It was opened as a small lunch wagon in 1895 and was one of the first places in the U.S. to serve steak sandwiches. According to Louis' Lunch, the hamburger was created in 1900 in response to a customer's hurried request for a lunch to go. In 1917, Louis moved the business into a square-shaped brick building that had once been a tannery.
In 1975, the restaurant was moved four blocks down to 263 Crown Street. Hamburgers cooked in the restaurant are made on the original cast iron vertical gas broilers from 1898, and the toast is made in a 1929 Savory Appliance Radiant Gas Toaster.
Lassen's restaurant is recognized in the Library of Congress as the origin of the hamburger, but other claimants and detractors exist.
Louis Lassen (1865 - March 20, 1935) was a "blacksmith by trade and preacher by vocation" and immigrated to New Haven from Denmark in 1886. He became a food peddler, selling butter and eggs from a wooden cart. He purchased a home at 45 Elliot Street and stored his cart in a shed in the backyard. In 1895, he began adding lunch items to his cart.
A local businessman dashed into the small New Haven lunch wagon one day in 1900, and he pleaded for a lunch to go. According to the Lassen family, the customer exclaimed "Louie! I'm in a rush, slap a meatpuck between two planks and step on it!". Lassen placed his own blend of ground steak trimmings between two slices of toast and sent the gentleman on his way, so the story goes, with America's alleged first hamburger being served. In 1917, Lassen moved into a square brick building that had once been a tannery.
Louis' Lunch was forced to move to make way for development in 1975, so it moved two blocks down to 263 Crown Street in New Haven. In the 1970s, Ken Lassen added cheese spread to the hamburger. The fifth generation of Lassens' owns and operates Louis' Lunch today
According to wikipedia