In 1937, Diokno graduated as valedictorian of his high school class at De La Salle College, Manila, now called De La Salle University (DLSU), and went on to take a bachelor's degree in commerce, also at DLSU. He initially wanted to pursue a career in mechanical engineering, but eventually compromised with his parents to take up commerce since his parents wanted a major that featured legal studies. He realized he enjoyed the legal courses the most and decided to take up law once he finished his undergraduate studies. He graduated from college summa cum laude at age 17 due to repeated acceleration. Diokno took the CPA board examinations in 1940—for which he had to secure special dispensation, since he was too young at 18 and only in his second year of law school at the University of Santo Tomas (UST). He topped the CPA with a grade of 81.18. Unfortunately, since Diokno was too young when he passed the CPA exam, he could not earn a proper license until he was around 21 years old, which made him even more determined to follow his father's footsteps and gain plenty of insight in law school like his other siblings.