[1] In 1843, Sainsevain was granted Rancho Cañada del Rincon en el Rio San Lorenzo near Santa Cruz, California by Governor Manuel Micheltorena, and in the Fall of 1843 he built one of the first sawmills in the valley of the San Lorenzo River, in association with Charles Roussillon (also known as "Rochon"), another Frenchman.

In 1844, he was granted permission to start a flour mill with a daily capacity of 75 fanegas on the Guadalupe River in San Jose.

Sainsevain, Roussillon, Antonio Sunol, and Amador, with help from twenty-five Indians, mined gold at Don Pedro's Bar on the Tuolumne River.

Sainsevain and Roussillon soon had enough of mining and returned to Stockton in 1849 to open a store supplying the California Gold Rush miners.

[4] In 1859, Sainsevain sold Rancho Cañada del Rincon en el Rio San Lorenzo, and with his brother, Jean Louis, bought the El Aliso vineyards in Los Angeles from their uncle, Jean-Louis Vignes (also known as "Don Luis Del Aliso").

In 1874 the Sainsevains purchased land in Hawker Canyon four miles east of Etiwanda and built a large stone house and a reservoir.

In 1868, Jean Louis Sainsevain was awarded a contract by the City of Los Angeles to create a new domestic water system.