Deng Yang was a descendant of Deng Yu, an official who lived in the early Eastern Han dynasty. He was from Xinye County, Nanyang Commandery, which is present-day Xinye County, Henan. At a young age, Deng Yang was already famous in Luoyang, the imperial capital of the Cao Wei state in the Three Kingdoms period. His fame put him on par with other contemporaries such as Xiahou Xuan, Zhuge Dan and Tian Chou. He held the positions of Gentleman of Writing, Palace Gentleman, and Prefect of Luoyang during the reign of Cao Rui, the second Wei emperor. However, he was later dismissed from office for engaging in superficial and fame-seeking behaviour. In 239, following Cao Rui's death, Cao Fang became the new emperor. However, as Cao Fang was still too young at the time, Cao Shuang and Sima Yi ruled as regents on his behalf. Through some political manoeuvres, Cao Shuang removed Sima Yi from power and became the sole dominant figure in the Wei government, while Sima Yi claimed to be ill and remained at home. During this time, Cao Shuang appointed Deng Yang as the Administrator of Yingchuan Commandery and later as a Chief Clerk under him. As one of Cao Shuang's close aides, Deng Yang was subsequently promoted to Palace Attendant and Master of Writing. While Deng Yang was in office, he engaged in corrupt and nepotist practices. For example, he once gave an official appointment to Zang Ai in return for Zang Ai giving him one of his father's concubines as a mistress. At the time, there was a saying in Luoyang which mocked Deng Yang: "Deng Xuanmao gives out official positions in return for women." In 244, Deng Yang and Li Sheng advised Cao Shuang to launch a military campaign against Wei's rival state, Shu, to boost his fame and authority in Wei. Cao Shuang ultimately lost the Battle of Xingshi against the Shu forces and his prestige fell as the Wei forces suffered heavy casualties in the campaign. In 249, while Cao Shuang and the emperor Cao Fang were away at the Gaoping Tombs, Sima Yi used the opportunity to stage a coup d'état against Cao Shuang and seized control of Luoyang. Cao Shuang surrendered to Sima Yi after the latter promised him that he and his family would be unharmed if he gave up his powers as regent. Later, Sima Yi broke his promise as he had Cao Shuang and his supporters arrested, charged with treason, and executed along with their families.
