O'Neill is originally from East Lansdowne, Pennsylvania, one of seven children. After whitewater kayaking as a child, he began climbing as a teenager, scaling the cables of the Brooklyn Bridge and climbing in Yellowstone. He left Temple University after one semester and worked at the Old Faithful Gift Shop in Yellowstone, and in Yosemite. He has lived in Boulder, Colorado since 2000. He climbs buildings, including the Chicago Tribune Tower, without ropes. In 2001, along with Dean Potter, O'Neill set the-then speed record for the "Nose" climb on El Capitan in Yosemite National Park, climbing it in three hours and twenty-four minutes.
O'Neill has been featured in several climbing films including Return2Sender, Front Range Freaks, and most recently 180 Degrees South. He also produced "Return2Sender", which won Best Climbing Film from the Alpine Club of Canada. He narrated and co-wrote the non-fiction comedy "Across the Atlas", about an adventure in Morocco that turned out to be less epic than the participants hoped. One of his comedy personas is "Dr Steven "Death Zone" Clark", in his "Mallory Revisited" play.
With Dennis “DJ” Skelton he co-founded a non-profit organization for disabled outdoor sport athletes, Paradox Sports, in 2007. His brother Sean broke his back jumping off a bridge into the Mississippi River and is still a climber. Paradox Sports is a 501(c)(3) nonprofit organization that improves people's lives by creating an adaptive sport community built to inspire. Currently, O'Neill is the executive director of Paradox Sports
- Abbey Smith. Timmy O'Neill Archived 2011-05-26 at the Wayback Machine. Climbing Magazine.
- Fahim, Kareem (7 June 2008). "The People Who Aspire to Great Heights, Literally, Hand Over Hand". New York Times. Retrieved 23 May 2011.
- Oksenhorn, Stewart (7 May 2009). "Boulder's Timmy O'Neill: Laughing all the way to the summit". The Aspen Times. Retrieved 23 May 2011.
- Brown, Zak (25 May 2007). "The Climbing Comedian: O'Neill one of Boulder's biggest characters". Daily Camera. Retrieved 23 May 2011.
- Knight, Wendy (6 January 2006). "Adventure Film Festivals: Would-You-Dare Movies". New York Times. Retrieved 23 May 2011.
- Soutar, Gaby (24 February 2007). "High point of the year". Scotsman. Retrieved 23 May 2011.