Alexander Garland Pendleton was born in Virginia on March 27, 1819, to William Garland Pendleton and Mary Glenn Alexander. His father was an attorney and public official in Richmond, and from 1832 to 1836 he served as proctor of the University of Virginia. Alexander enrolled at UVA in 1834 and spent the next three years studying math, natural philosophy, ancient languages, and civil engineering. He listed Charlottesville as his residence in the student catalogue and received permission from the faculty to remain at the university during vacations.

After leaving UVA, Pendleton married Selina Christina Dickson on September 25, 1840, in Norfolk, Virginia. They had seven children together: Selina D., born in 1844; Rose B., born in 1846; Edwin Conway, born in 1847; Alexander G., born in 1848; Frances, born in 1850; William G., born in 1855; and Douglas, born in 1857. The family primarily lived in and around Washington, DC. Pendleton played an active role in local Democratic politics, serving as a delegate to the party’s district conventions in the 1840s and 1850s.

On August 14, 1848, Pendleton joined the faculty of the United States Naval Observatory in Washington, D.C., as a Professor of Mathematics. He held his position at the Observatory throughout the Civil War. Chronic ophthalmia, however, eventually forced him to give up observing and begin drafting charts and publications for ships preparing to sail. During his time at the Naval Observatory, Pendleton “rendered effective and valuable services,” and colleagues remembered him as “an efficient officer” with “a beautiful character.”

In February 1865, while traveling home from his office, Pendleton began feeling unwell and stopped at a house a few miles outside of Washington, DC. He passed away on February 16, and was buried in Oak Hill Cemetery. His wife Selina died twenty years later, on June 24, 1885, and was interred at his side.