Nelson Dellis was originally inspired to improve his memory after seeing the decline of his grandmother's memory due to Alzheimer's disease, and entered his first memory competition in 2009. He has since placed in a number of competitions. In the US Memory Championship, he took third place in 2010, first place in both 2011 and 2012, second place in 2013, and first place again in 2014. He also placed 7th in the 2012 World Memory Championships.

Dellis holds a number of memory records, including the US national record for memorizing a deck of shuffled cards in 63 seconds as well as the US national record for memorizing the most digits in 5 minutes, with 303 digits memorized. He is also ranked 15th in the world for memorizing a deck of cards, with a time of 40.65 seconds, the fastest time for an American in an international competition. He currently ranks 21st in the world as a memory athlete.

Dellis was featured in the 2012 documentary Ben Franklin Blowing Bubbles at a Sword: The Journey of a Mental Athlete. He also appeared in the Science Channel program Memory Games in July 2013, which covered the 2013 US Memory Championships. He has also been interviewed regarding memory training on Today, The Dr. Oz Show, andNightline.