During John Brown's raid on Harpers Ferry in 1859, Steuart personally led a detachment of militia, and, as the prospect of civil war drew closer, he was among those who lobbied unsuccessfully for Maryland to secede from the Union.

In 1861, at the start of the Civil War, Steuart left his home state of Maryland and joined the Confederacy, though at 71 years of age he was by then considered too old for active service.

[11][12] Some of the militia regiments, such as the 51st, and some members of 39th, broke and ran under fire, but the 5th and 27th held their ground and were able to retreat in reasonably good order having inflicted significant casualties on the advancing enemy.

The British took significant losses, including their commanding officer Major General Robert Ross, and, lacking the strength to take the city of Baltimore, they eventually withdrew.

Steuart was soon promoted to lieutenant-colonel of the 5th Regiment,[14] and after the war he trained as a lawyer, being listed in the Baltimore City Directory of 1816 as Attorney-at-Law.

Also a guest was the Swedish feminist and activist Fredrika Bremer, who wrote in a letter to her sister Agathe: "Late in the evening I sat in the most beautiful moonlight with Miss Dix on the veranda of General Stuarts' [sic] house, looking towards the shining river and the wide Chesapeake Bay, listening to the story of her simple yet remarkable life".

Hinks duly gave Steuart the order, writing that he should "hold yourself with your command, or such portion thereof as you may deem necessary, in readiness to march at a moment's warning, fully armed and equipped for active service".

[30] In the 6th ward artillery was used, and a pitched battle fought on Orleans St between Know Nothings and rival Democrats, raging for several hours.

[30] In 1857, fearing similar violence at the upcoming elections, Governor Thomas W. Ligon ordered Steuart to hold the First Light Division, Maryland Volunteers in readiness.

[31] Ligon carried a "painful sense of duty unfulfilled" owing to the violence of the previous year, and was determined to maintain order.

In 1828 Steuart served on the board of managers of the Maryland State Colonization Society, of which Charles Carroll of Carrollton, one of the co-signers of the Declaration of Independence, was president.

[35] In 1846 his father James Steuart died, and he inherited Maryland Square, his family's mansion in the western suburbs of Baltimore.

[36] In 1859 Steuart's militia participated in the suppression of John Brown's raid on Harpers Ferry, an abortive attempt to ignite a slave rebellion.

Steuart argued for "the invalidity of Lincoln's election, because of the negro votes cast and counted for him in the states of New York, Ohio, and Massachusetts".

[41] On April 19 Baltimore was disrupted by riots, during which Southern sympathizers attacked Union troops passing through the city by rail, causing what were arguably the first casualties of the Civil War.

[42] Perhaps knowing this, and no doubt aware that public opinion in Baltimore was divided, Governor Thomas Holliday Hicks refused to order out the militia.

In a letter of the same date he wrote to Governor of Virginia John Letcher stating that he was: Steuart's efforts to persuade Maryland to secede from the Union were in vain.

Arrests of Confederate sympathizers soon followed, and General Steuart fled to Charlottesville, Virginia, after which much of his family's property was confiscated by the Federal Government.

[48] Union troops were quartered in Steuart's mansion and Jarvis Hospital was soon erected on the grounds of the estate, to care for Federal wounded.

Baltimore resident W W Glenn described him as living in constant fear of capture: General Steuart corresponded regularly with a friend, Sally J. Newman, in Hilton, Va. during course of the war.

[63] The Grace Medical Center continues to flourish today, and forms an important part of the modern neighbourhood, which still retains the name of Steuart Hill.