According to the Lions' official website, the centre linked up with the Golden Lions squad last week, however his contract was only finalised on Wednesday.
He will thus be available for selection with immediate effect.
Golden Lions backline coach Swys de Bruin is thrilled to welcome 25-year-old Mnisi to the Johannesburg union, having noticed his talents from an early age.
“I coached Howard at junior level, while at the Sharks, and I am excited to welcome him to the Golden Lions,” said De Bruin.
“He is certainly a player that I feel will fit in well within our backline structures and he will no doubt contribute a great deal to our overall team objectives.”
Mnisi gained valuable Currie Cup experience at Griquas and also played for the Cheetahs during the 2014 Super Rugby tournament.