A drawn-out wrangle over who should oversee Schmid's majority stake had prevented MobilCom from receiving a €100m (£63.6m) government loan needed to keep the company afloat.
Schmid and the German Federal Economic Ministry elected Helmut Thoma, a MobilCom supervisory board member, as the sole trustee.
The agreement could pave the way for a €7bn bailout of the wireless operator, one of six to have won a licence to operate third-generation mobile broadband networks in Germany.
France Télécom, which owns 28.5% of MobilCom, has agreed in principle to take charge of the €7bn bailout, but only if Schmid has no further say in running the company.
MobilCom said it would announce a restructuring programme in the coming days.
Rumours are circulating in the German press that the company may abandon the 3G market to focus on its traditional business of reselling voice services to consumers.