She worked as a deputy prosecuting attorney in Honolulu before becoming a United States magistrate judge on August 2, 1999.

She was nominated to fill the seat vacated by Judge Helen W. Gillmor, who assumed senior status in 2009.

[4][5] This makes her the first Japanese American federal judge confirmed during the Obama Administration.

[3] On August 8, 2023, Kobayashi partially blocked Hawaii's ban on firearms in public places.

[7] [8] On September 6, 2024, the 9th circuit largely reinstated Hawaii's firearm ban, [9] [10] and denied en banc on January 15, 2025.