The Atomic Knights appeared off and on in issues of Strange Adventures in the early 1960s, beginning with #117 (June 1960). Created by John Broome and Murphy Anderson, they were a band of heroes living in the post-apocalyptic future of 1992. Following the catastrophic Hydrogen War of 1986, a petty tyrant named the Black Baron ruled a small section of the Midwest with an iron fist. He was opposed by Sgt. Gardner Grayle and the Atomic Knights, who wore medieval suits of armor that were impervious to the Baron's energy weapons, having been irradiated in the war. The other Knights were twins Wayne and Hollis Hobard, Bryndon Smith, the last scientist left on Earth, and brother and sister Douglas and Marene Herald.
The 15 Atomic Knights stories in Strange Adventures generally dealt with post-holocaust recovery, as the Knights would fend off menaces and attempt to rebuild the area around their homebase of Durvale, though they also managed to travel to Los Angeles, Detroit, New Orleans, New York, and Washington, D.C. In all, there were 15 early-1960s Atomic Knights stories; their last appearance in Strange Adventures came in issue #160 (January 1964).
The Atomic Knights concept then lay dormant for several years, until Cary Bates used the Knights as guest-stars in the mid-70s series Hercules Unbound, beginning with #10 (April-May 1977). Hercules, Kamandi and the Atomic Knights all inhabited the same universe, one in which the Great Disaster had taken place (references to 1986 became less and less frequent as that date actually approached). The entire Great Disaster concept has since been declared to be out of continuity in the DC Comics universe. The Atomic Knights themselves were revealed to be the dream of Gardner Grayle in DC Comics Presents #57, where Superman attempts to prevent Grayle from causing a nuclear war.
The original Atomic Knights stories were reprinted in Strange Adventures #217-231. Their appearances in Hercules Unbound and DC Comics Presents have not been reprinted.
After a mildly successful career as a superhero, Grayle took a job at S.T.A.R. When he received a premonition from the goddess Cassandra, Grayle proceeded to use his new technical know-how to build his atomic armor. He then participated in the Crisis on Infinite Earths as one of the Forgotten Heroes who contacted Darkseid to enlist his help against the Anti-Monitor.
Afterward, he appeared in the Outsiders (vol. 2), a team he joined up until its disbanding. The Outsiders would later reform (in vol. 3), but were considered fugitives after being framed for the slaughter of a Markovian village (home country to Outsider Geo-Force). At first he hunted the team down but was ultimately convinced of their innocence and was able to clear their name to the rest of the world. He also helps the Outsiders battle a vampiric infestation of the underground nation of Abyssia. With the Outsiders, he also fell in love with fellow Outsider Windfall and they were seen together at Geo-Force's wedding.
Beyond just his Atomic Knight armor, Gardner also had the power to see the future.
In the post-Infinite Crisis mini-series The Battle for Blüdhaven, Gardner Grayle is the leader of an underground band of new Atomic Knights operating within the destroyed city of Blüdhaven, working with an organization named the Roundtable towards helping citizens harassed by the organization known as S.H.A.D.E. and the new Black Baron (a former pimp and drug dealer who gains metahuman powers after the Blüdhaven destruction and is later defeated by the golem known as Monolith). There are roughly 125 Atomic Knights, with the main Knights being Grayle, Marene and Doug Herald, Bryndon, and Wayne and Hollis Hobard (which would seem to indicate this team is directly based on the original group from the 1960s), who are shown posing as refugees with the help of advanced cloaking technology, and using armor with numerous powers including the ability to record and analyze complex data and fire powerful ballistic and nuclear blasts. At the end of the series, Captain Atom obliterates the remains of Blüdhaven, making way for the departure of S.H.A.D.E. operatives. After this, the Knights are seen entering an underground lush environment through a bunker named Command-D, reminiscent of their access to the post-nuclear world of their original incarnation. Command-D is the bunker that Kamandi and his Grandfather the original OMAC lived.
In 52, the existence of a new Multiverse is revealed. These Earths are originally carbon copies of the New Earth created at the end of Infinite Crisis.
Based on comments by Grant Morrison, this alternate universe is not the original setting of the 1960s stories.