[6] The San Gabriel Valley was first inhabited by the Tongva whose traditional lands extended throughout the area of modern-day greater Los Angeles.

[7] While Juan Rodríguez Cabrillo first discovered California for Spain in 1542, Spanish colonization did not begin until 1769 with the Portolá expedition.

The mission utilized the lower portion of the valley for raising cattle and farming while the uplands, including Monrovia, remained untouched.

[10] On April 16, 1841, Hugo Reid, a naturalized Mexican citizen from Great Britain, was provisionally granted ownership of Rancho Santa Anita, which included most of present-day Monrovia to the west of Norumbega Drive, and received a fee simple title to the land in 1845.

[12] In 1884, William N. Monroe purchased a 120-acre plot from Baldwin in present-day Monrovia that encompassed both sides of White Oak Avenue (now Foothill Boulevard).

In 1886, Monroe partnered with Edward F. Spence, John D. Bicknell, and James F. Crank to form a 120-acre town centered around Myrtle Avenue and present-day Colorado Boulevard; the first sale of townsite lots began on May 17, 1886.

[17][18] Monrovia was connected to Los Angeles by rail in 1888 through the efforts of the San Gabriel Valley Rapid Transit Railroad Company.

The same year, the Pacific Electric was opened, providing transportation to and from Los Angeles, making it possible for Monrovia homeowners to work in Los Angeles.In 1905, Carnegie funds became available and, with the help of the Board of Trade (forerunner to the Chamber of Commerce) and the Monrovia Women's Club, a bond issue was passed to purchase the Granite Bank Building to be used as a city hall, and to acquire property for a public park.

The small airstrip claimed to have had 12,000 paying customers use the airfield in 1932 and on May 19, 1938, the first airmail flight took off from Monrovia Airport.

Apart from usage by Riley Brothers, TWA Captain and former airshow pilot Kalman Irwin, and Pancho Barnes, the airfield is well known for its use as a movie-filming location.

Other films shot at the airfield include 20,000 Men a Year, The Great Plane Robbery, and most notably, The Big Noise, featuring Laurel and Hardy.

The 35-acre airfield, used as a runway as well as an airplane repair and storage service, was forced to close in 1953 after being sold to Consolidated Engineering Corporation for redevelopment as a result of increased land values.

In 1937, Patrick McDonald opened a food stand on Huntington Drive (Route 66) near the old Monrovia Airport called "The Airdrome" (hamburgers were ten cents, and all-you-can-drink orange juice was five cents); it remained there until 1940, when he and his two sons, Maurice and Richard, moved the building 40 miles (64 km) east to San Bernardino to the corner of West 14th Street and 1398 North E Street, renaming it "McDonald's".

According to the United States Census Bureau, the city has an area of 35.5 square kilometers (13.7 sq mi).

[23] In 2024, a mother black bear and her young cubs began regularly visiting a home in Monrovia situated close to the mountains to cool-off and play in the property's swimming pool.

According to the 2010 United States Census, Monrovia had a median household income of $71,768, with 9.8% of the population living below the federal poverty line.

[47] Original Tommy's, Trader Joe's, Green Dot and Naked Juice are based in Monrovia.

Monrovia has a "Technology Corridor,"[48][49] which includes AeroVironment, Tanner Research, Parasoft, Xencor, and ITT Deep Space Division.

[53] Monrovia's main roads include Foothill Boulevard and Huntington Drive (historic Route 66).