In 1861, he founded the Continental Iron Works in Greenpoint, Brooklyn, which built ironclad warships for the United States Navy during the American Civil War, most notably USS Monitor, which successfully neutralized the threat from the Confederate ironclad CSS Virginia in the Battle of Hampton Roads in 1862.

Rowland was described as an energetic and inventive leader, who designed many of his own company's machine tools, accumulating more than fifty patents in the course of his lifetime.

[8] In 1859, Samuel Sneden, a builder of wood-hulled steamboats in Greenpoint, Brooklyn, was offered a contract by a regular customer to build an iron-hulled ship.

[10][11] In January 1861, however, Sneden & Co. abruptly failed[10] after submitting a low bid for the construction of a large-diameter water mains across the Harlem River.

[10][13] Rowland renamed the yard the Continental Iron Works,[8][10] and the water mains contract would later be completed by the new firm.

[2][4] The establishment of the Continental Works coincided with the outbreak of the American Civil War in April 1861, and later that year, Rowland became associated with New York engineer John Ericsson, who had just secured a contract with the United States Navy for the construction of a radically new type of ironclad warship with low freeboard and a revolving gun turret.

[14] Ericsson subcontracted Rowland for construction of the hull, and the new warship—later commissioned as USS Monitor—was launched at the Continental Works in just 101 days.

[17] At one point in 1864, Rowland attempted to sell the Continental Works and go into early retirement,[18] but this did not eventuate, and instead he would remain the company's president for most of his life.

After this war, the company increasingly focused on the manufacture of gas mains and large welded water pipes.

[2][19] The Rowlands had four children: a daughter, Caroline Attwater, who died in infancy, and three sons, Thomas Jr., Charles and George,[2][28] the first two of whom mentioned following their father into the family business.

"[30] He was active as a philanthropist,[19] notably pioneering the practice in New York of granting a half-day holiday to employees on Saturday afternoons,[30][31] which he extended to his own workforce with no reduction in pay.