The honor is the result of more than three decades of lobbying from Cushing's descendants and Civil War history fans who began a letter-writing campaign to then-Wisconsin Sen. William Proxmire.
Posthumous awards are typically only granted within two years of the act of heroism for which the honoree receives the award, but Congress granted an exemption last December for Cushing.
Cushing nearly received his honor in 2012, after a lengthy Army investigation approved his nomination and the award was announced. But Virginia Sen. James Webb stripped the amendment, which would have allowed the award, from the National Defense Appropriations Act in December.
"It is impossible for Congress to go back to events of 150 years ago to make individual determinations in a consistent, equitable and well-informed manner," Webb said at the time.
"While one would never wish to demean any act of courage, I believe that the retroactive determination in one case could open up an endless series of claims. The better wisdom would be for Congress to leave history alone."
A new amendment was added to the NDAA in 2013, and was passed by Congress in December. It was reviewed by the Department of Defense and approved by the president this summer.
Born in 1841 in Wisconsin and raised in New York, Cushing graduated from the U.S. Military Academy at West Point just two months after the start of the Civil War.
At Gettysburg, 1st Lt. Cushing commanded 110 men and six cannons. On July 3, 1863, the final day of the battle, his men stood their ground in the face of Picket's Charge. His battery was reduced to a working field piece, and Cushing was shot in the stomach and the shoulder.
Historians say Cushing should have ordered his battery to withdraw so replacements could take their place, but he ordered them to the front lines instead. He was shot in the head and killed instantly.
"Refusing to evacuate to the rear despite his severe wounds, he directed the operation of his lone field piece continuing to fire in the face of the enemy," the White House announcement said. "With the rebels within 100 yards of his position, Cushing was shot and killed during this heroic stand. His actions made it possible for the Union Army to successfully repulse the Confederate assault."
Cushing will join more than 1,500 soldiers from the Civil War who have received the Medal of Honor, most recently Cpl. Andrew Jackson Smith in 2001.
Army Command Sgt. Maj. Bennie G. Adkins and Army Spc. Donald P. Sloat (posthumous) will also receive the honor at the September 15 ceremony.
Sloat was killed in action in Vietnam on Jan. 17, 1970, when he picked up a live grenade and used his own body to shield his fellow soldiers from the blast.
Adkins, who plans to attend the ceremony, will receive the honor for actions on his second of three tours to Vietnam, in 1966, when he braved enemy fire to bring wounded soldiers to safety.