The two firms will assist in investigating concerns about the fitness to practise of dentists and other dental professionals such as orthodontists.
Their appointments follow a competitive tendering exercise.
GDC chief executive and registrar Duncan Rudkin said: “We put all the firms competing for some of our work through a challenging process to ensure that we get the best value for the GDC’s money and the quality of legal services we need in order to deliver an efficient and effective fitness to practise system.”
The two firms will work alongside the GDC’s in-house legal team of four lawyers.
Earlier this month Mills & Reeve also hired three new lawyers from Kennedys to its Cambridge healthcare team.
John Chapman, a former Mills & Reeve partner who left to head Kennedys’ NHS trust team, returned to Mills & Reeve with Lucy Johnston and Caroline Deeley, who are both former trainees at the firm.