Kim Yo-jong was born as the daughter of Kim Jong-il and his wife Ko Yong-hui on 26 September 1987.  She and Kim Jong-un are said to have a close relationship, due to shared isolated years while studying together in Switzerland from 1996 to 2000, and in North Korea, "where social and emotional isolation appears to have been a defining force in their early lives."  She may have studied at the Kim Il-sung Military University after her return.  She also studied computer science at Kim Il-sung University where she is said to have studied with Kim Eun-gyong, the daughter of Japanese abductee Megumi Yokota. 
In January 2015, she reportedly married Choe Song, the second son of government official Choe Ryong-hae.  However, South Korean government sources have since denied the accuracy of these reports. Kim Yo-jong was pregnant and expecting a child in May 2015. The father, not identified, is thought to be a fellow alumnus of Kim Il-sung university and either an official at the Workers Party's Room 39 or working at a military unit responsible for guarding the country's leader. 
She was first spotted at a photo session for participants at the 3rd Conference of the Workers' Party of Korea (WPK) in September 2010, when she stood next to her father's personal secretary and alleged mistress Kim Ok. 
Kim Yo-jong was given much publicity during the funeral service for Kim Jong-il in December 2011, when she appeared several times alongside her brother Kim Jong-un or leading funeral processions of central officials, despite not even being a funeral committee member, and she was never named. She was then reportedly given a position under the National Defence Commission at the beginning of 2012 as tour manager for Kim Jong-un,  but has not appeared in news reports except for November 2012, when the Korean Central Television showed her accompanying Kim Jong-un at a military riding ground.  She was officially mentioned for the first time on 9 March 2014, as she accompanied her brother in voting for the Supreme People's Assembly. Kim Yo-jong was identified as a "senior official" of the WPK Central Committee. 
In October 2014, she was reported to have possibly taken over state duties for her ailing brother while he underwent medical treatment. 
On 28 November 2014, she was named vice director of the Workers Party's Propaganda and Agitation Department. Since March 2015, she has been its head. She also holds a vice ministerial post but her portfolio is not known.  She regularly accompanies Kim Jong-un on his "field guidance" trips. 
On 24 July 2015, Kim Yo-jong was put in charge of North Korea’s Propaganda and Agitation Department (PAD). As the vice director of the country’s propaganda arm, replacing Kim Ki-nam. She was said to be the driving force behind the development her brother’s cult of personality, modelled after that of their grandfather, Kim Il-sung.  This would help explain changes in the way state policies are depicted in the media, as well as differences in reporting.  She replaced Thae Yong-ho, a North Korean defector and former diplomat, said in 2017, and that Kim Yo-jong organised all major public events in North Korea. Kim Yo-jong was said to have encouraged her brother to present an image of a "man of the people" with, for example, rides on fairground attractions and his friendship with the basketball star Dennis Rodman. 
In 2017, Kim Yo-jong was made an alternate member of politburo.  Her ascension to the country's supreme governing body may indicate that she is Kim Jong-un’s replacement for his aunt, Kim Kyong-hui, who has not played an active role in his regime.