The 40-strong London operation is run by former CW managing director Adrian Roxan, who retains the same title.
The company's Leeds base was formerly part of Citigate Dewe Rogerson. With ten staff, it is run by MD Claudine Moyle.
Key's 25-strong Birmingham office, headed by MD Christine Arthur, will join the new agency in two months.
Roxan said there were no plans to merge Incepta's other two PR subsidiaries - Citigate Public Affairs and Citigate Dewe Rogerson - into the new operation.
Key was acquired by Incepta in 2000 in a £6.8m deal (PRWeek, 24 November 2000). The early steps to a merger took place shortly after., when Incepta merged its existing Midlands corporate PR operation with Key's Birmingham office.
Roxan said: 'There have been no redundancies. This is a move to retain all our staff and create a stronger brand.'