The district is located in the San Gabriel Valley and contains most of the Hispanic-majority areas in that region.

From January 3, 2023, following the 2020 redistricting cycle, the district is currently represented by Democrat Gil Cisneros.

It takes up part of eastern Los Angeles County.

The 31st, 35th, and 38th are partitioned by Whittier Narrows Recreation Area, N Lexington-Gallatin Rd, N Durfree Ave, E Thienes Ave, E Rush St, N Burkett Rd, Cunningham Dr, Eaglemont Dr, Oakman Dr, Arciero Dr, Grossmont Dr, Workman Mill Rd, Bunbury Dr, Fontenoy Ave, Ankerton, Whittier Woods Circle, Union Pacific Railroad, San Gabriel Freeway, N Peck Rd, Mission Mill Rd, Rose Hills Rd, Wildwood Dr, Clark Ave, San Jose Creek, Turnbull Canyon Rd, E Gale Ave, Pomona Freeway, Colima Rd, E Walnut Dr N, Nogales St, E Walnut Dr S, Fairway Dr, E Valley Blvd, Calle Baja, La Puente Rd, S Sentous Ave, N Nogales St, Amar Rd, Walnut City Parkland, San Bernardino Freeway, Fairplex Dr, Via Verde, Puddingstone Reservoir, McKinley Ave, N Whittle Ave, Arrow Highway, Fulton Rd, and Foothill Blvd.

The 31st district takes in the cities of El Monte, West Covina, Baldwin Park, Azusa, San Dimas, La Verne, Duarte, South El Monte, Industry, La Puente, Bradbury, Irwindale, and Covina, the south sides of the cities of Glendora and Monrovia, and the census-designated places Avocado Heights, North El Monte, South San Jose Hills, West Puente Valley, Valinda, Mayflower Village, South Monrovia Island, Vincent, Citrus, and Charter Oak 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 Formerly, from 2003 to 2013, the district was located in Los Angeles County.