Kennedy made the formal announcement in a video released by his campaign.
Gun violence will be front and center of the pair's campaign - a personal issue for both Kennedy and Joy. Joy's son was shot and killed in June, and Kennedy's father, Sen. Robert Kennedy, was fatally shot in 1968.
"When conversations started around this campaign...it provided an opportunity for me to turn some of that pain into purpose, and it also presents an opportunity to continue some of that work that I've been doing over the last year and a half," Joy said.
Joy, who is 44 years old, is executive director of CHANGE Illinois. He has a long history of political activism, including helping push for automatic voter registration in Illinois.
Kennedy said Joy will be a key asset in getting voters, particularly millennials, involved.
"They need somebody who has a proven track record of standing of up to corrupt government...there aren't many of those in Illinois...he is a citizen servant," said Kennedy.
Kennedy faces an uphill battle. Gubernatorial candidate, billionaire J.B. Pritzker, has support of the Cook County Democratic Party. Kennedy said he is not worried about that, and will be focusing on getting people involved at the grass roots.