The 18th Street gang was formed in the 1960s. According to Sergeant Richard Valdemar, of the Los Angeles County Sheriff's Department, the 18th Street gang had its origins in racial prejudice. During the 1960s, the Clanton Street gang, a well-established Hispanic street gang, was in its second generation. Youth in the local neighborhood wanted to join the gang, but the membership of Clanton Street was limited to those youth who were American citizens from a pure Hispanic background. Youth who were undocumented immigrants or of mixed ancestry were not allowed to join the gang. Although turned away by Clanton Street, these juveniles still participated in criminal activities. Like many young juvenile offenders, they were arrested and sent to juvenile detention facilities. While in these facilities, their membership to Clanton Street was denied. As a result, these youth from the Clanton Street neighborhood formed their own gang. A young man, nicknamed "Glover," was in a detention facility, and started to recruit mixed-race youth to form a gang. These youth were the original members of 18th Street. According to Sergeant Valdemar, the young man who started the Clanton Street Throw-aways lived on 18th Street, just four blocks away from the Clanton Street gang. The new gang adopted the name of his street. This street was located an area now known as the Rampart section of Los Angeles.

The 18th Street gang was the first Hispanic gang to break the racial membership barrier. This willingness to step across racial lines allowed rapid and unchecked growth in the gang's membership, which was largely composed of immigrants and multi-racial youths. 18th Street also recruited heavily from the populations of illegal immigrants entering the United States from Mexico and South/Central America. Although primarily composed of Hispanics, some cliques of 18th Street have recruited African Americans, Asians, Caucasians, and Native Americans. Some tagger crews who operated within 18th Street territory were also actively recruited, but only if the crews had a reputation for violence. For example, West Side 18th Street "jumped in" 50 members of a tagger crew known as KWS, Kings With Style. KWS members were known by law enforcement to be involved in robbery, assaults, drive-by shootings, and murder.

Uniquely, the 18th Street gang members, though primarily turf-oriented, also travel to other areas and states for membership recruitment and illegal activities. This tendency to travel explains 18th Street's wide-scale presence in many different states. However, while 18th Street members have dispersed the gang through relocation and targeted recruitment, the overall research on gangs still supports the idea that most gangs are indigenous to their areas of origination. Very few gangs send members out of state to recruit new members and to establish new cliques or sets of their gang. The 18th Street gang was the first Hispanic street gang to do this. Law enforcement intelligence supports the assumption that some of these recruits have been sent out with a specific purpose. At one time, intelligence indicated that "tagger crews" that were jumped in to 18th Street became "tax" collectors, enforcers, and narcotics distributors.