The following information is from LA Avenue.com.
When Spanish occupation of California began, an exploratory expedition of more than 60 persons led by Gaspar de Portola moved north through the area now known as Los Angeles. They camped by a river where fertile soil and availability of water for irrigation impressed members of the party. Father Juan Crespi, who accompanied the group, saw the location as having all the requirements for a large settlement. He named the river El Rio de Nuestra Senora la Reyna de Los Angeles de Porciuncula, which means "The River of Our Lady the Queen of the Angels of Porciuncula. "
In September 1771 Father Junipero Serra and a group of Spaniards founded the San Gabriel Mission as the center of the first "community" in an area inhabited by small bands of Gabrielino Indians.
Twelve years after Portola's trek, which began in San Diego and ended in Monterey, a company of settlers called "Los Pobladores" were recruited in the states of Sonora and Sinaloa in Mexico. Their mission, under authority of Governor Felipe de Neve, was to establish pueblos in the name of the king of Spain.
On September 4, 1781, the Pobladores, a group of 12 families - 46 men, women and children led by Captain Rivera y Moncada - established a community in the area discovered by Portola, and named it El Pueblo de Nuestra Senora la Reyna de Los Angeles de Porciuncula, after the nearby river. Over time, the area became known as the Ciudad de Los Angeles, "City of Angels," and on April 4, 1850 became the City of Los Angeles.
California was ruled by Spain until 1822 when Mexico assumed jurisdiction. After a two-year period of hostilities with Mexico beginning in 1846, the area came under U.S. control. In 1848 the Treaty of Guadalupe Hidalgo made California a United States territory.
The County of Los Angeles was established on February 18, 1850 as one of the 27 original counties, several months before California was admitted to the Union. It derived its name from the area known as Los Angeles, already a large community, and made it the designated "seat" of County government.
On April 1, 1850 the people of Los Angeles County asserted their newly won right of self-government and elected a three-man Court of Sessions as their first governing body. A total of 377 votes were cast in this election. In 1852 the Legislature dissolved the Court of Sessions and created a five-member Board of Supervisors. In 1913 the citizens of Los Angeles County approved a charter recommended by a board of freeholders which gave the County greater freedom to govern itself within the framework of state law.