 In 1841 he worked at Vignes' sawmill near San Bernardino.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.