Osuna played an important role in the establishment of the pueblo of San Diego (population 150) and was elected alcalde.
In addition, Juan Osuna became a Justice of the Peace and was administrator of the San Diego Mission.
In 1845, Governor Pio Pico awarded him full title to the two square league Rancho San Dieguito.
Osuna was considered a fine alcalde but was also a gambling man and lost some of his land to pay off bad debts.
[4][5] With the cession of California to the United States following the Mexican-American War, the 1848 Treaty of Guadalupe Hidalgo provided that the land grants would be honored.