Lamb became a professional poker player in 2004. In September 2006, he won the United States Poker Championship $500 No Limit hold 'em 400-player event for a prize of $60,000. Another notable championship was his victory in the May 11, 2010 Spring Championship of Online Poker $109 No Limit hold 'em event. He outlasted a field of 4,204 competitors for a prize of $66,003. In 2011, he dominated the World Series of Poker on his way to the Card Player Player of the Year Award.
At the 2011 WSOP Main Event, Lamb was chip leader after Day 1-B, which was the second of four Day 1 starts in the tournament. He was also the overall chip leader after Day 2-B (of two Day 2 starts), entering Day 3 with 551,600 chips. On Day 6, he took the lead in the Player of the Year race and finished the day in second place in chips out of 57 remaining players. On Day 8, he qualified for the 2011 November Nine in fifth place with 20,875,000 chips. Lamb went on to finish in third place, earning $4,021,138.
As of 2017, his live poker tournament winnings exceed $8,200,000.