The deal adds connections to more than 40 African locations for travelers from the US.
'United continues to demonstrate our commitment to Africa, starting three brand-new flights to the continent this year alone.' said United vice president Patrick Quayle.
Airlink is bidding to fill a void left by South African Airways, which recently underwent bankruptcy proceedings and is flying to limited destinations.
United plans to start flights from Washington to Nigeria and Ghana later this year, alongside a route between New York and Johannesburg.