In June 1822 the Scottish merchant Hugh McCulloch landed at San Diego from the brig John Begg. He was the founding partner of the business McCulloch Hartnell & Co which for three years from 1 January 1823 had a monopoly of the Hide and Tallow Trade with the Californian missions. His story is told here https://durnesstocalifornia.blogspot.com. Was he the first Scotsman to ever set foot in California?
In 1814 a Scotsman by the name of John Cameron jumped ship. He later changed his name to Gilroy. Here's some info from the city of Gilroy California web page.
John Cameron was born in a southern district of Inverness-shire, Scotland in 1794. At 19, he left home, hiring aboard a British trading ship which arrived, in 1814, at what was then the Spanish harbor of Monterey. It’s unclear when the young sailor changed his surname to Gilroy, his mother’s maiden name, but he was baptized Juan Bautista Gilroy at the Mission San Carlos Borromeo del Rio Carmelo, and before long was conversant with Spanish. Eventually, he journeyed further inland to Rancho San Ysidro where he made barrels for the Rancho’s owner, Don Ygnacio Ortega. In 1819 Gilroy received permission from the Viceroy of Spain to remain in California and to marry. Two years later Gilroy wed the Ortegas’ daughter, Maria Clara, at Mission San Juan Bautista. Of their 17 children, 9 survived to adulthood. In 1833, Gilroy became a naturalized citizen of Mexico; Don Ortega died in this same year. The Governor of Mexico granted that Rancho San Ysidro’s land be divided equally amongst Ortega’s three adult children and their spouses. On his portion of the rancho, Gilroy raised cattle, wheat and also ran a soap-making business. Known for his hospitality and community spirit, he served as alcalde (mayor) of San Ysidro, and in 1846 was appointed Juez de paz (Justice of the Peace) for the district. Gilroy died in July of 1869, and left many descendants who live in the area. He is buried in the Old St. Mary Cemetery.
You can find several entries concerning Gilroy in Bancrofts History of California (though he labels him as an Englishman)
My answer here, to the question Which Irishman worked at Mission Santa Cruz? , mentions another. (From A Memorial and Biographical History of the Coast Counties of Central California) :
John Gilroy in 1814, and John Rose, in 1818 were Scotchmen
So third at best.