The 24-year-old signs for Usuthu after four seasons in the Betway Premiership with the Natal Rich Boys.
He previously turned out for KwaZulu-Natal ABC Motsepe League team Ethekwini Coastal FC.
Reflecting on his switch, Mhlongo explained that he is fulfilling a life-long dream by joining Usuthu.
He told the club's website: "I feel happy and honoured to play for a club I grew up wanting to represent — a club like AmaZulu Football Club.
"I kindly ask for the supporters' backing; I am ready to give my all to help the team achieve its objectives.
"I look forward to playing alongside my new teammates and working with coach Arthur Zwane and his technical team.
"I would also like to thank my previous club for everything they did for me."
