Monrovia is an unincorporated community and census-designated place in Frederick County, in the U.S. state of Maryland. As of the 2020 census it had a population of 2,702. Though a post office was established in Monrovia on January 15, 1833, it is not known that the town was named Monrovia in the 1830s. In the 1820s, the town was known as "Hall To…Monrovia is an unincorporated community and census-designated place in Frederick County, in the U.S. state of Maryland. As of the 2020 census it had a population of 2,702. Though a post office was established in Monrovia on January 15, 1833, it is not known that the town was named Monrovia in the 1830s. In the 1820s, the town was known as "Hall Towne," likely after Nicholas Hall, a founder of New Market. When the name Monrovia was applied to the town is unknown, however why it was named such was either after the U.S. President James Monroe, or perhaps for an event described by Reverend Herbert Austin Cooper in 1997...