US captains in every Olympic sport met at the Olympic Village and voted to award the honor for Friday's ceremony to Lomong, a sensitive choice given criticism of Chinese foreign policy over the conflict in Darfur.
"This is the most exciting day ever in my life," Lomong said. "It's a great honor for me that my teammates chose to vote for me.
"The opening ceremony is the best day and the best moment of Olympic life. I'm here as an ambassador of my country and I will do everything I can to represent my country well."
Lomong, 23, was kidnapped from his family by the Janjaweed militia and taken hostage. He and other youths escaped and spent three days on the run before crossing the border into Kenya and being taken to a refugee camp.
He spent years there just fighting to survive and famously paid five Kenyan shillings to watch a black and white television telecast of the 2000 Sydney Olympics.
He said seeing US track star Michael Johnson win gold sparked his dream of becoming an Olympian.
Lomong was adopted by a US foster family, changed his citizenship to American and will race in the 1,500 meters at Beijing.
China has close ties with Sudan, as one of the main buyers of the African nation's oil and a key investor in its economy, and rights groups have accused Beijing of not doing enough to try and resolve the conflict in Darfur.
The United Nations has said that 300,000 people have died in Darfur and that more than 2.2 million have been displaced since 2003. The Sudanese government puts the number of fatalities at 10,000.