In his new role as Mother's strategy director, Mildenhall will also lead Mother's new-business effort, as well as look after two as-yet-unnamed major accounts.
He will fill the gap left by Clare Dobbie, the senior strategist who left Mother last month. She was brought in to replace the departing managing director, Will Orr, in May, but only four months on she joined The Gap as its European marketing director.
Mildenhall left TBWA in March. He was linked to a top job at TBWA\New York, but after conversations broke down he quit the London agency to attend Harvard Business School's Advanced Management Programme.
Stef Calcraft, a partner at Mother, said: "Mildenhall brings unrivalled flair and charisma - a huge asset to all those who work with him."
Mildenhall added: "My role will be to spread the Mother gospel to the four corners of the creative world."