What is the longest city name in the United States?

It's probably the full name of Los Angeles which is El Pueblo del la Reina de Los Angeles which means The Town of the Queen of the Angels.

I don't know about the L.A. thing, but there is a town called "Truth or Consequences" in New Mexico which is 19 characters (not including the spaces).

The longest place name in the United States (45 letters) is Chargoggagoggmanchauggagoggchaubunagungamaugg, a lake in Webster, Massachusetts. It means "Englishmen at Manchaug at the Fishing Place at the Boundary" and is sometimes facetiously translated as "you fish your side of the water, I fish my side of the water, nobody fishes the middle". The lake is known to Americans as Lake Webster. The longest hyphenated names in the U.S. are Winchester-on-the-Severn, a town in Maryland, and Washington-on-the-Brazos, a notable place in Texas history.

https://en.wikipedia.org/wiki/Longest_word_in_English#Place_names

How about Rancho Santa Margarita, CA, at 22 characters with spaces? That's actually a city name, not a place, town, or lake. Truth or Consequences is impressive, but still one character shorter with spaces.

http://www.cityofrsm.org/

LA does hold claim to the longest city name in the United States. The full name of Los Angeles as it was founded September 4, 1781, by Spanish governor Felipe de Neve is: El Pueblo de Nuestra Señora la Reina de los Ángeles de la Porciúncula (The Village of Our Lady, the Queen of the Angels of Porziuncola). There is some question about the legitimacy of this name, which may have, through a series of misinterpretations and inflations, been corrupted from the actual name authorized in writing in 1781, "La Reina de Los Angeles". Cf. Theodore E. Treutlein, "Los Angeles, California: The Question of the City's Original Spanish Name," Southern California Quarterly 55, no. 1 (Spring 1973): 1-7.

Historian Doyce B. Nunis, Jr., has traced the longer name to the histories written by the Franciscan missionaries, especially Francisco Palóu, who wished to play up the region's connections to their order. Pool, Bob, "City of Angels' First Name Still Bedevils Historians." Los Angeles Times (March 26, 2005), Sec. A-1.