He was in the Union Army in the Civil War, serving as a captain at Antietam in the U.S. 1st Cavalry Regiment and during the Gettysburg Campaign. Reno was wounded at Kelly's Ford in Virginia on March 17, 1863, and was given the brevet rank of major for gallant and meritorious conduct. That same year, he married Mary Hannah Ross of Harrisburg, who would bear him one son, Robert Ross Reno. They owned a farm near New Cumberland, Pennsylvania in Cumberland County. When she died in 1874, Reno was in the field in Montana and rode all night to Fort Benton to request leave to attend her funeral. The request was denied.
Reno was present at the 1864 battles of Cold Harbor, Trevilian Station and Cedar Creek. After serving in a variety of staff positions, he was brevetted lieutenant colonel in October. In December, Reno became brevet colonel of the 12th Pennsylvania Cavalry, later commanding a brigade against John Mosby's guerrillas. On March 13, 1865, he was brevetted brigadier general for "meritorious services during the war."
In 1866 Reno was ordered to Fort Vancouver, in the Pacific Northwest. He served as acting assistant inspector general of the Department of the Columbia.
Reno was promoted to major and in December 1868, joined the 7th Cavalry at Fort Hayes, Kansas. Later, he was transferred to Fort Abraham Lincoln, in the Dakota Territory, and would accompany George A. Custer on his Sioux campaign in 1876.
The plan quickly fell apart when Northern Cheyenne and Lakota Native American warriors, rather than fleeing as the cavalrymen expected, poured out of the village to meet Reno's attack. Reno ordered his troops to dismount and form a skirmish line, but that was quickly outflanked by hundreds of Indians and Reno fell back into the timber along the river.
As Indians began to infiltrate the timber, Reno realized that position could not be held either, and he led a disorganized, every-man-for-himself scramble across the river and up the bluffs on the other side where the cavalrymen set up a defensive position on what is now called Reno Hill. By this time 40 of Reno's 140 men already had been killed, 13 were wounded and 16 had been left behind in the trees (although most of these abandoned men would manage to rejoin Reno.) Bloody Knife, perhaps the foremost of the Arikara and Crow scouts attached to the 7th Cavalry was shot through the head as he stood next to Reno. Most of the other scouts slipped away and escaped.
Benteen soon arrived at Reno's position with his three companies, and McDougall's company came along with the supply train shortly afterward. Sporadic fire continued to be directed at the hill, but heavier gunfire off to the northeast made it clear Custer was engaged in a raging battle. It is still hotly debated why Reno and Benteen did not press forward and attempt to join forces with Custer. Captain Weir, commander of D Troop under Benteen, impatient to join the battle going on three miles away, led his men to the high bluffs now called Weir Point, but he soon came under heavy attack and was barely able to regain the safety of Reno's position.
Having destroyed Custer's force—although Reno and his men had no way of knowing about what happened to Custer—the Indians then took up the high ground above Reno Hill and poured down constant fire on the exposed soldiers until dark. The firing resumed at dawn and continued until late in the afternoon, when the soldiers saw the distant village being broken up and the Indians moving off. The next morning, the 27th, the survivors were able to move closer to the river, where General Alfred Terry and Colonel John Gibbon and their forces found them. Thirteen of Reno's soldiers were awarded the Medal of Honor for their bravery in the battle.
After the Battle of the Little Big Horn, Reno was assigned the command of Fort Abercrombie where, in December 1876, he was charged with making unwanted advances toward the wife of another officer of the 7th Cavalry, Captain James Bell, while Bell was away from the fort. An Episcopal minister, the Rev. Richard Wainwright, was staying with the Bells, and became concerned enough about Reno's behavior to persuade Capt. Bell to file charges against Reno for immoral conduct.
What really happened is unknown, but Reno certainly displayed public drunkenness on several occasions and denied Rev. Wainwright permission to preach at the fort. On the other hand, Bell had been on detached service during the Battle of the Little Big Horn, when his company had been wiped out in a debacle which many were blaming on Reno. Complaints of public indecency were filed with the commander of the Seventh, Colonel Samuel D. Sturgis, who forwarded the complaints, but dismissed any particular fault of Reno's. (Most of the incidents had occurred at parties or on holidays when other officers also had been drinking.) Reno was ordered to surrender command and report to a board of inquiry at St Paul. The Army board recommended dismissal, but President Rutherford B. Hayes commuted this to suspension from rank and pay for 2 years.
Responding to charges of cowardice and drunkenness at the Little Big Horn, Reno later demanded and was granted a Court of Inquiry. The court convened in Chicago in January 1879, and called as witnesses most of the surviving officers who had been in the fight. Enlisted men later stated they had been coerced into giving a positive report to both Reno and Benteen. The court reporter in contacting General Nelson Miles, head of the Army, wrote that the entire inquiry was a whitewash. While the court did not sustain any of the charges against Reno, neither did it single him out for praise. Later, in public requests for the trial transcripts, pages were missing and the writing was in the hands of two different people and not the one secretary.
In 1880, however, he was court-martialed a second time for conduct unbecoming an officer because of his drinking. He was supported by his commanders, but nevertheless was convicted and dismissed from the service. Reno moved to Washington D.C., where he was hired by the Bureau of Pensions as an examiner. He married a government clerk named Isabella Ray in January 1884, but she left him after a few months. When his son married in Nashville to a whisky heiress, Reno wrote that he was too busy to attend the wedding. In reality, he could not afford the train fare. Reno offered to write his memoirs, but the New York Weekly Press rejected the offer. When he submitted the portion of his diary concerning the Battle of the Little Big Horn, it was returned unpublished. (It later was published posthumously.)
In 1967, a US military review board reviewed the original documents and testimony of Reno's 1880 court martial and reversed the decision. His "general discharge" status to was changed to "honorable". Major Reno was originally buried in an unmarked grave in Washington's Oak Hill Cemetery. His remains were re-interred in 1967 in the Custer National Cemetery, on the Little Bighorn Battlefield.