Born in Modesto, California, to John Howard Aldrich DeChambeau and Janet Louise Druffel, DeChambeau moved to Clovis, east of Fresno, at age seven. He attended Clovis East High School and won the California State Junior Championship at age 16 in 2010. DeChambeau graduated in 2012 and accepted a scholarship to Southern Methodist University in Dallas, Texas, majoring in physics.
Waste Management Phoenix Open: Claimed his second runner-up finish at the Waste Management Phoenix Open, one shot behind Hunter Mahan. Had birdie opportunities on the last three holes but unable to catch Mahan. Previously lost in a playoff to Troy Matteson just down the road at Grayhawk GC in the 2009 Frys.com Open. Had he won at TPC Scottsdale, would have become the first Sponsor Exemption to do so since Jason Gore (2005 84 Lumber Classic). Lee Westwood later won in June as a Sponsor Exemption at the St. Jude Classic presented by Smith & Nephew.
Lowry elected to turn professional the week after his Irish Open triumph, meaning that he would miss out on the opportunity to play in the Walker Cup in September 2009. He made his professional début on 28 May at the European Open, where he shot 78 in the first round to end the opening day almost at the back of the field, and went on to miss the cut following a second round 73.