The district covers several cities in the San Gabriel Valley and the Gateway Cities region of Los Angeles including the cities of: Diamond Bar, Industry, La Habra Heights, La Mirada, Montebello, Norwalk, Pico Rivera, Santa Fe Springs, Walnut, and Whittier.

A small portion of Pomona and the Orange County city of La Habra are also included in the district.

Most of the cities have a Hispanic majority, primarily Mexican, while Diamond Bar, Hacienda Heights, Industry, Rowland Heights and Walnut have an Asian-American majority, primarily Chinese.

The 38th district takes in part of the cities of Downey and Pomona, part of the census-designated place Avocado Heights, the cities of Whittier, Montebello, Norwalk, Pico Rivera, Diamond Bar, La Mirada, Walnut, Santa Fe Springs, La Habra Heights, and Industry as well as the census-designated places Hacienda Heights, East Whittier, South Whittier, West Whittier-Los Nietos, Rose Hills, and Rowland Heights 1962 • 1964 • 1966 • 1968 • 1970 • 1972 • 1974 • 1976 • 1978 • 1980 • 1982 • 1984 • 1986 • 1988 • 1990 • 1992 • 1994 • 1996 • 1998 • 2000 • 2002 • 2004 • 2006 • 2008 • 2010 • 2012 • 2014 • 2016 • 2018 • 2020 • 2022 From 2003 through 2013, the district consisted of most of the San Gabriel Valley in Los Angeles County.

Due to redistricting after the 2010 United States census, the district has moved east and south east in Los Angeles County and includes much of the previous areas as well as Bellflower and Artesia.