According to the United States Census Bureau, the CDP has a total area of 1.0 square mile (2.6 km2), 98.56% of it land and 1.44% of it water.
This region experiences warm (but not hot) and dry summers, with no average monthly temperatures above 71.6 °F. According to the Köppen Climate Classification system, McArthur has a warm-summer Mediterranean climate, abbreviated "Csb" on climate maps.
The 2010 United States Census reported that McArthur had a population of 338. The population density was 332.7 people per square mile (128.5/km²). The racial makeup of McArthur was 217 (64.2%) White, 0 (0.0%) African American, 15 (4.4%) Native American, 0 (0.0%) Asian, 0 (0.0%) Pacific Islander, 98 (29.0%) from other races, and 8 (2.4%) from two or more races. Hispanic or Latino of any race were 119 persons (35.2%).