Chronos was David Clinton, a small time Physics professor from Gotham City, who developed the technology to travel in time.
Clinton's peers spurned him for his claims of discovering time travel, which cost him both his tenure and reputation. However, Clinton came through. He developed time traveling technology and incorporated it into a 'Chronosuit'. He used the device to collect rare relics from the past. However, he initially only took stuff that wouldn't be missed, lest he change the timestream. Clinton went to the past and attempted to steal Batman's utility belt from the Watchtower, but he was thwarted by John Stewart, Wonder Woman, and Batman. He jumped into a time vortex, but the League chased him.
Clinton arrived in the Old West (1879), where he was jumped by a group of men working for the local crime lord, Tobias Manning. They stole his time traveling device, which Manning used to steal advanced weaponry from the future, and then take over the town of Elkhorn. Clinton was kept behind bars for nearly six months ― until the League finally caught up with him ― to teach Manning how to use the technology he couldn't understand. When the Leaguers, Ohiyesa Smith, Bat Lash, Jonah Hex, and El Diablo defeated the outlaws, Clinton managed to retrieve his belt and fled in a time portal, followed by the Green Lantern, Wonder Woman, and Batman again.
Clinton had initially been a pushover, mocked by his time traveling theories, and constantly thrashed by his verbally abusive wife, Enid. However, his newfound power drove him insane, and he became a megalomaniacal sociopath. He now traveled back to his time (the future) and asserted himself as Lord Chronos, the undisputed master of space and time. He no longer cared about time paradoxes, and started collecting all sorts of relics from the past. He took over Gotham and used the city as a big storage space to collect his memorabilia. He also made himself king, escorted by a very much tamed Enid, now scared of Chronos' temper and mood swings. He enlisted the Joker's Jokerz as his Royal Guard, and endowed them with special enhancements.
Then, he pitted them against the Leaguers, right when they arrived from their time travel. However, the Jokerz failed due to treason within the group. Chronos ruthlessly disposed of the traitor ― Chucko ― by leaving him in the past, seconds before the dinosaurs were made extinct by a meteorite. Due to Chronos' tampering with the past, the timeline started to unravel, so the League and the JLU tracked him down to reprogram his belt and restore time. Chronos fled to the Beginning of Time, to reset it and make himself a god. However, he was promptly intercepted by John Stewart and Batman, who reprogrammed the time-belt. This reversed Chronos' doings, trapping him in a time loop, right before everything started.
Chronos debuted in Atom #3 (October-November, 1962), and was a creation of Gardner Fox, Gil Kane and Murphy Anderson. David Clinton was petty crook who believed his failures were a consequence of his bad timing. While serving sentence in prison, Clinton studied and trained himself to synchronize his actions with clockwork precision. When he got out of prison, he donned a flamboyant costume and dubbed himself Chronos, the Time Thief. He also developed various time related gimmicks (sundial-shaped hovering platform, razor-sharp hands, and explosive hourglasses) to meet his goals. Chronos pitted himself against the Atom (Ray Palmer), who thwarted him on every occasion. Eventually Chronos developed devices that could actually influence the local flow of time and tamper with time, such as a time machine. Chronos soon began using his time traveling abilities to become rich, but he was eventually sent to prison. He later sold his soul to Neron, who bestowed upon him the inherent ability of time traveling. Chronos ended up losing touch with timeline and dwindled into oblivion.