He was born on September 30, 1839 as Asa Bird Gardner without the "i", he added the "i" in 1884. in New York City. He was born in Fraunces Tavern, where his father and uncle were barkeepers. His father later ran the Philadelphia Hotel.

He graduated A.B. from City College and LL.B. from New York University School of Law in 1860. He was admitted to the New York City Bar Association and began private practice as an attorney.

He was commissioned a first lieutenant in the 31st New York Infantry Regiment on May 27, 1861, at the outbreak of the American Civil War. He was mustered out of service on August 7, 1861 and was commissioned a captain in the 22nd New York on May 31, 1862, and was honorably mustered out on September 5, 1862. He was again commissioned a captain in the same regiment on June 18, 1863, and was again mustered out on July 24, 1863. During that time he served in the Gettysburg Campaign and received the Medal of Honor on September 23, 1872, for "distinguished service performed during the war while serving as Captain 22nd New York State Militia".

Later in the war, Gardiner was commissioned a first lieutenant in the Veterans Reserve Corps on February 11, 1865, and served as regimental adjutant until he was honorably mustered out of service on August 13, 1866. Gardiner was brevet a captain on March 13, 1865, for "gallant and meritorious service during the war".

After the end of the Civil War, Gardiner was commissioned a second lieutenant of the 9th Infantry Regiment of the Regular Army and was promoted to first lieutenant on February 14, 1868. He transferred to the 1st Artillery Regiment on April 3, 1869. Gardiner was promoted to the rank of major on August 18, 1873, and served as a Judge Advocate for 15 years until he retired from the Army on December 8, 1888.

Gardiner served from 1874 to 1878 as Professor of Law at the United States Military Academy at West Point. By Act of Congress, the Academy established a Department of Law in 1874, with a senior Judge Advocate General as its first Professor of Law. U.S. Secretary of War William W. Belknap appointed Gardiner to the post, and he became the first lawyer to teach law at the Academy. Gardiner initiated the entire law curriculum, including study of the Lieber Code and a textbook he himself wrote.

In 1875, while still at West Point, Gardiner was chosen by President Ulysses S. Grant to be the presiding judge advocate general at the Whiskey Ring court-martial of Gen. Orville E. Babcock, Grant's personal secretary. The civilian grand jury that had already convened refused to turn over its evidence, however, and the court-martial adjourned; Babcock was later acquitted.

In 1878, a commission reviewed the court-martial of Fitz John Porter, who had been drummed out of the Army in 1863 for his actions at Second Bull Run. Chairman John M. Schofield appointed Gardiner as Recorder, but he "took upon himself the role of a judge advocate in a court-martial," contesting evidence favorable to Porter. The Commission ultimately re-instated Porter.

In 1880, the sole black cadet at West Point, Johnson Chesnut Whittaker, was allegedly assaulted by three fellow cadets, but the white commanders at the Academy decided in an inquest that he had faked the attack. After a year of public outcry including the attention of the United States Congress, Whittaker was court-martialed, with Gardiner as prosecutor. The result was Whittaker's expulsion. The verdict was overturned in 1883 by President Arthur; Whittaker was still expelled from West Point on the grounds of failing a exam. In 1995 President Clinton presented a commission to Whittaker relatives.

In 1884, Gardiner was selected for another high-profile prosecution, that of his superior, Brig. Gen. David Swainn, the Judge Advocate General of the Army; Swainn was convicted.

After retiring from the Army, Gardiner pursued the private practice of law in New York City and became active in the Tammany Hall political machine, the major faction of the New York City Democrats. A history of the society calls him a "simon-pure Democrat" who followed his father and grandfather's participation in the Tammany Society, where in 1901 he was elected a sachem.

In November 1897, Gardiner was elected on the Democratic ticket New York County District Attorney, and took office on January 1, 1898, together with the first elected officers of the Consolidated City of New York. Gardiner was notorious for having said, "Reform be damned" when confronted with calls to confront the corruption of Tammany Hall. In December 1900, Governor Theodore Roosevelt removed Gardiner from office.

Among the beneficiaries of Gardiner's attitude was saloonkeeper Frank J. Farrell, who is said to have opened three hundred pool halls (in reality fronts for bookmakers) after his friend took office, building a fortune that he would use to bring the New York Yankees to town.

In 1916, the U.S. Army revoked his Medal of Honor on the grounds that there was insufficient evidence of his heroism. Gardiner refused to return his medal and was controversial until the day he died in 1918.

Gardiner was active in numerous military and hereditary societies including the Society of the Cincinnati, the Military Order of the Loyal Legion of the United States, Sons of the Revolution, Society of Colonial Wars and the Veteran Corps of Artillery of the State of New York. In the Society of the Cincinnati he served as the President of the Rhode Island Society and as General Secretary of the National Society for many years.

He died on May 24, 1919 of a stroke of apoplexy suffered the previous Saturday afternoon.