He was the first mayor to be born in the city of Washington, graduating from Yale University in 1837 and returning to the capital to practice law in the early 1840s.

Thus when mayor William Winston Seaton declined to run for a sixth term in 1850, Lenox was the heir apparent — although because of his young age (only 33), he was dismissed by many residents of the city, particularly when the popular former mayor Roger C. Weightman announced his intention to seek the office again.

In June 1861, after the occupation of Alexandria by Union forces during the Civil War, he moved to Richmond where he organized other "refugees" from DC, Maryland and Delaware.

Upon returning to Washington in October 1863 to settle the estate of a deceased relative, he spoke openly in contempt of the Union and was arrested and imprisoned by his old friend, General Winfield Scott.

He spent the next 20 months in the prison at Fort McHenry and was released in October 1865, half a year after the war ended.