Glendora is a city in the San Gabriel Valley in Los Angeles County, California, 26 miles east of Los Angeles. As of the 2020 census, the …Glendora is a city in the San Gabriel Valley in Los Angeles County, California, 26 miles east of Los Angeles. As of the 2020 census, the population of Glendora was 52,558. The city is located along the fabled Route 66.