He was the Chief State Councillor of the Korean Joseon from September 1418 to December 1418 and the father of Queen Soheon and father-in-law to King Sejong.
His father, Sim Tŏk-bu, participated in the Wihwado retreat alongside General Yi Sŏng-gye and later became a merit subject (개국공신; 開國功臣; lit.
Sim On passed the lower literary examination of gwageo in 1386 at the age of 12, though records of his government service only appear after Joseon's establishment in 1392.
Meanwhile, Taejong had abdicated and become king emeritus (상왕; 上王), although he still retained significant power, with important decisions still requiring his approval.
In the aftermath, many officials from the Ministry of Military Affairs, including Kang Sang-in and Minister Park Seup, were imprisoned.
After a few days of interrogation, Kang Sang-in was pardoned due to his status as a merit subject of the kingdom's founding and was sent back to his hometown.
According to Park Seup's confession, Sim On, Kang Sang-in, and others had discussed splitting military authority was impossible, which led to reporting directly to Sejong.
However, when Sejong and Queen Soheon's son King Munjong rose to the throne, Shim-On was absolved of his crimes, and posthumously reinstated to the office of Chief State Councilor.
The Shim family continued to prosper, with Sim-On's second son Sim Hwe becoming Chief State Councilor during King Sejo's reign.