During the Second Seminole War (1837–1838), McArthur was promoted to the temporary rank of lieutenant and placed in command of a small craft.

[1] The von Humboldt left Panama on May 21, 1849 and took 102 days to arrive at San Francisco, the first 46 of which were spent getting to the Mexican port of Acapulco.

[1] Among the four hundred passengers on von Humboldt were Collis P. Huntington, the future president of the Southern Pacific Railroad and San Francisco Society portrait painter Stephen W. Shaw.

[2] Upon reaching San Francisco, the Ewing and the USS Massachusetts were hampered from progress in the survey due to desertions of their crews who joined the gold rush, including a mutiny when crew members rowing into the city from the Ewing threw an officer overboard in an attempt to desert to flee to the gold fields.

[3] They managed to survey Mare Island Strait[2] before steaming to Hawaii to obtain crewmen from Hawaiian monarch King Kamehameha III.

[1] They returned to San Francisco in the spring of 1850, with the coastal survey of northern California beginning on April 3, 1850, and continuing up the coast of Oregon to the mouth of the Columbia River.

On August 1, 1850, while still in Oregon,[clarification needed] McArthur purchased a 1/16 interest in Mare Island for $468.50,[2] then returned to San Francisco later that month to prepare charts and write reports.

The scenery is beautiful and in some places and some points of view the grandest that the eye ever beheld.Lieutenant Blunt who accompanied him on the expedition even made a land claim on behalf of himself, McArthur and Bartlett.