William Edward Petty Hartnell (April 24, 1798 – February 2, 1854), later known by his Spanish name Don Guillermo Arnel, was a merchant, schoolmaster, and government official in California.

He held several public roles during the Mexican era and after the American Conquest of California, notably serving as the official translator at the Monterey Constitutional Convention.

Hartnell attended the College of Commerce in Bremen, Germany and then went to Chile in 1819 to work in the Santiago office of John Begg & Co., a firm where another uncle, Edward Petty, had helped him secure a job.

With the waning of Spanish power in the region, the Scottish trading company gradually expanded its commercial activities from Valparaíso to Callao and other ports on the northern Pacific coast of South America.

The pair arrived in Monterey in 1822, the first foreign traders to arrive there since Mexico obtained its independence from Spanish rule in 1821, and after obtaining permission from Governor Pablo Vicente de Solá to permit them as British subjects to do business and live in Alta California they visited the Californian missions and signed an exclusive trading contract with the majority of them for three years from the beginning of January 1823.

After learning that his benefactor uncle, from whom he had stolen money, was in straitened circumstances and poor health Hartnell suffered a crisis of conscience and began drinking heavily.

In 1832 Hartnell was elected leader of the Compañia de Extranjeros, a militia of foreign residents, formed at the request of Commandant Agustín V. Zamorano to maintain order in Monterey during unrest and disturbances.

[8] He was also appointed Inspector of the Missions in January 1839 but encountered opposition from administrators, Zacatecan padres, military officials and rancheros which led to his resignation from the post in September 1840.

[9] Whilst carrying out his inspection at Mission San Rafael Arcángel, he was arrested by his former pupil Mariano Guadalupe Vallejo who accused him of interfering in matters concerning the northern frontier without his consent.

However, Hartnell's brother-in-law Pablo de la Guerra was able to secure him a job establishing a treasury in the city of Yerba Buena (modern San Francisco).

A regular correspondent of Hartnell's during this period was Robert Crichton Wyllie, with whom he conspired to obtain land grants in California for British colonists, though the idea never came to fruition.

[1] During this period he shared an office with then lieutenant William Tecumseh Sherman, who had accompanied Hartnell and Thomas O. Larkin in rowing out to meet the SS California when it arrived in Monterey in 1849.