Reardon was a free-solo climber, a small fraternity of people who climb significant rock without ropes, harnesses, or partners. He came late to the sport, taking it seriously only in his thirties, but once he committed he produced a body of work that drew the attention of the wider climbing world. At Joshua Tree National Park in California he made a thousand individual solo ascents in thirty days, including a hundred first-ascent solos at grades up to 5.12. He onsight-soloed Outrage, a 5.13a route at Boney Bluffs in the Santa Monica Mountains, meaning he climbed it first try without rehearsal. He onsighted 214 routes in England and 240 in Ireland, including a first ascent at Ailladie in County Clare. National Geographic named him Adventurer of the Year for 2005.

His most striking single feat may have been on the Palisade Traverse, a high-altitude scramble across the Sierra Nevada in California that links a series of fourteen-thousand-foot peaks. The previous unroped traverse had taken twelve days. Reardon, working alone, completed it in twenty-two hours. The achievement was largely off the climbing media's radar at the time because it was difficult to verify and difficult to film, but other climbers understood what it meant. It required not only the ability to move efficiently over technical terrain at altitude, but the willingness to stay focused for an entire day and into the next, without backup, with consequences for any error that did not allow recovery.

By 2007 he had begun to film a documentary about solo climbing on Irish sea cliffs. Dohilla, on the rough north shore of Valentia Island, was the kind of location he favored: low-altitude, geologically interesting, and quietly serious. He climbed his line that afternoon, came down, and was photographed standing at the base of the cliff. A rogue wave, larger than the swell pattern around it, swept up the shelf and took him into the water. The Valentia lifeboat launched within minutes, joined by a rescue helicopter, cliff rescue teams, and local people from Knightstown and Portmagee. The search ran along the coast until dark, and again the next day, and again the day after. He was not found. The Atlantic, which had carried the first transatlantic cable ashore at Foilhommerum a few miles south, kept him.

His daughter was thirteen at the time. The climbing community lost not only one of its most prolific soloists but one of its more thoughtful voices; Reardon wrote and spoke publicly about the choices involved in his discipline, the discipline itself, and the cost when those choices failed. The Irish climbing community in particular treated his death gently, with a memorial plaque eventually placed at the Iveragh coast. He had loved Ireland enough to come back, and to choose its quieter cliffs over the more famous walls he could have visited anywhere in the world. Valentia Island, where so many ocean stories converge, holds one more.