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Ygnacio Palomares received the 22,340-acre (90 km²) land grant Rancho San Jose from Governor Juan Bautista Alvarado in 1837. The land included the present day cities of Pomona, Claremont, La Verne, San Dimas, and Glendora.

Like many cities in the San Gabriel Valley, Glendora was established on previously remote agricultural land when the area became connected to the outside world upon the completion of the Santa Fe Railway's main transcontinental line from Los Angeles to Chicago in May 1887. Located at the foot of the San Gabriel Mountains, Glendora was started on approximately 300 acres (120 ha) that were subdivided and sold by George D. Whitcomb in late March 1887. On the first day of sale, 300 lots were sold. Whitcomb was the founder of the Whitcomb Locomotive Works in Chicago and Rochelle, Illinois and had moved to California in the early 1880s. He devised the name Glendora by combining the name of his wife, Leadora Bennett Whitcomb, with the location of his home in a glen of the San Gabriel Mountains. In December 1907, the development of Glendora got a boost when passenger service opened on a new extension of the Pacific Electric Railway's Monrovia-Glendora line which provided hourly one-seat ride service from downtown Glendora to the Pacific Electric Building at Sixth and Main in Downtown Los Angeles. Prior to its 1911 incorporation, Glendora's administrator officially occupied the office of President of Glendora.

The downtown area as it appears now is the product of years of renovation and maintenance by the city. The former opera house, movie theatre, Pacific Electric station, banks, hotels, grocery and department stores were converted into more modern commercial buildings. The original layout can be read about on the Downtown Glendora Historical Walk, by reading placards placed along Glendora Avenue (previously called Michigan Avenue). The original townsite was bounded by Sierra Madre Avenue on the north, Minnesota Avenue on the east, Ada Avenue and the railroad on the south, and Pennsylvania Avenue on the west.

Glendora used to be home to several military academies, which have since been converted into either churches or private school facilities. These academies included Brown Military Academy, now St. Lucy's Priory High School and Church of the Open Door on Sierra Madre, and Harding Military Academy, whose property is now home to North Glendora Private, a prestigious private community adjacent to Easley Canyon atop Glendora Avenue.

From 1960 to 1978, Glendora was home to Clokey Productions which produced 85 episodes of Gumby and 65 episodes of Davey and Goliath in town. In celebration of this history, Glendora hosted the first Gumby Fest in 2014 which brought thousands of people from around the country and Canada. In 2015 the 2nd GumbyFest was held at Citrus College over an entire weekend.

Several wildfires have affected the city in recent years, the most notable being the campfire triggered Colby Fire, which displaced hundreds of Glendora residents. A relief concert titled "Glendora Band Aid" was held shortly after the fire to help raise funds to assist the homeowners who lost their homes in the fire.

Glendora has an active Chamber of Commerce, established in 1903. The mission of the Glendora Chamber is to provide tools and resources to assist the business community in prospering and adapting to the economic climate, while growing membership and promoting local purchasing.

As of the census of 2010, there were 50,073 people, 16,819 households, and 12,866 families residing in the city. The population density was 2,581.5 inhabitants per square mile (996.8/km²). There were 17,145 housing units at an average density of 895.7 per square mile (345.9/km²). The racial makeup of the city was 75.1% White (57.0% Non-Hispanic White), 1.9% Black or African American, 0.7% Native American, 8.0% Asian, 0.1% Pacific Islander, 5.2% from other races, and 4.8% from two or more races. 30.7% of the population were Hispanic or Latino of any race.

There were 16,819 households, out of which 38.6% had children under the age of 18 living with them, 60.1% were married couples living together, 12.1% had a female householder with no husband present, and 23.5% were non-families. 19.1% of all households were made up of individuals, and 7.9% had someone living alone who was 65 years of age or older. The average household size was 2.88 and the average family size was 3.30.

In the city, the population was spread out, with 27.6% under the age of 18, 7.6% from 18 to 24, 29.1% from 25 to 44, 23.2% from 45 to 64, and 12.5% who were 65 years of age or older. The median age was 37 years. For every 100 females, there were 93.2 males. For every 100 females age 18 and over, there were 89.6 males.

According to the 2010 United States Census, Glendora had a median household income of $74,615, with 7.8% of the population living below the federal poverty line. More recently, the L.A. Times estimated the median household income at $81,336 based on updated figures from the U.S. Census and SCAG.

Notable people who live or have lived in Glendora, California include: