As a young man Gardner became interested in the socialist ideas being advocated by Robert Owen. Inspired by the New Harmony community established by Robert Dale Owen and Fanny Wright in Indiana, Gardner helped establish the Clydesdale Joint Stock Agricultural & Commercial Company. The plan was to raise funds and to acquire land in the United States.
In 1850 Gardner, his brother James Gardner, and seven others travelled to the United States. He purchased land and established a cooperative community close to Monona, in Clayton County, Iowa. Gardner returned to Scotland to help raise more money and to recruit new members.
Gardner used some of his funds to purchase the newspaper, the Glasgow Sentinel. Published every Saturday, the newspaper reported on national and international news. In his editorials, Gardner advocated social reforms that would benefit the working class. Within three months of taking control of the newspaper, circulation had grown to 6,500, making it the second-best selling newspaper in Glasgow.
In May, 1851, Gardner visited the Great Exhibition in Hyde Park, where he saw the photographs of Matthew Brady. Soon afterwards Gardner, who had always been interested in chemistry and science, began experimenting with photography. He also began reviewing exhibitions of photographs in the Glasgow Sentinel.
Gardner decided to emigrate to the United States in the spring of 1856. He took with him his mother, his wife, Margaret Gardner, and their two children. When he arrived at the Clydesdale colony, he discovered that several members were suffering from tuberculosis. His sister, Jessie Sinclair, had died from the disease and her husband was to follow soon afterwards.
Gardner decided to abandon the Clydesdale community and settle his family in New York. Soon afterwards he found employment as a photographer with Matthew Brady. Gardner was an expert in the new collodion (wet-plate process) that was rapidly displacing the daguerreotype. Gardner specialized in making what became known as Imperial photographs. These large prints (17 by 20 inches) were very popular and Brady was able to sell them for between $50 and $750, depending on the amount of retouching with india ink that was required.
In the 1850s Brady's eyesight began to deteriorate and began to rely heavily on Gardner to run the business. In February, 1858, Gardner was put in charge of Brady's gallery in Washington. He quickly developed a reputation as an outstanding portrait photographer. He also trained the young apprentice photographer, Timothy O'Sullivan.
On the outbreak of the American Civil War there was a dramatic increase in the demand for Gardner's work as soldiers wanted to be photographed in uniform before going to the front-line. The following officers were all photographed at the Matthew Brady Studio: Nathaniel Banks, Don Carlos Buell, Ambrose Burnside, Benjamin Butler, George Custer, David Farragut, John Gibbon, Winfield Hancock, Samuel Heintzelman, Joseph Hooker, Oliver Howard, David Hunter, John Logan, Irvin McDowell, George McClellan, James McPherson, George Meade, David Porter, William Rosecrans, John Schofield, William Sherman, Daniel Sickles, George Stoneman, Edwin Sumner, George Thomas, Emory Upton, James Wadsworth and Lew Wallace.
In July, 1861 Matthew Brady and Alfred Waud, an artist working for Harper's Weekly, travelled to the front-line and witnessed Bull Run, the first major battle of the war. The battle was a disaster for the Union Army and Brady came close to being captured by the enemy.
Soon after arriving back from the front Matthew Brady decided to make a photographic record of the American Civil War. He sent Gardner, James Gardner, Timothy O'Sullivan, William Pywell, George Barnard, and eighteen other men to travel throughout the country taking photographs of the war. Each one had his own travelling darkroom so that that collodion plates could be processed on the spot. This included Gardner's famous President Lincoln on the Battlefield of Antietam and Home of a Rebel Sharpshooter (1863).
In November, 1861, Gardner was appointed to the staff of General George McClellan, the commander of the Army of the Potomac. Granted the honorary rank of captain, Gardner photographed the battles of Antietam (September, 1862), Fredericksburg (December, 1862), Gettysburg (July, 1863) and the siege of Petersburg (June, 1864-April, 1865).
He also photographed Mary Surratt, Lewis Powell, George Atzerodt, David Herold, Michael O'Laughlin, Edman Spangler and Samuel Arnold after they were arrested and charged with conspiring to assassinate Abraham Lincoln. He also took photographs of the execution of Surratt, Powell, Atzerodt and Herold were hanged at Washington Penitentiary on 7th July, 1865. Four months later he photographed the execution of Henry Wirz, the commandant of Andersonville Prison in Georgia.
After the war Brady established Gardner own gallery in Washington. This included taking photographs of convicted criminals for the Washington police force. He also published a two-volume collection of 100 photographs from the American Civil War, Gardner's Photographic Sketchbook of the War (1866).
In 1867 Gardner became the official photographer of the Union Pacific Railroad. As well as documenting the building of the railroad in Kansas, Gardner also photographed Native Americans living in the area. Alexander Gardner died in Washington in 1882.