Cllr Barrie Cooper has joined the Mayor and Cllr Mieka Smiles.
The appointment follows the resignations of five former members of the Executive on Tuesday.
Cllr Cooper, who represents the Newport ward, will take on responsibility for the portfolios of Environment, Finance and Governance and Regeneration.
Cllr Smiles retains responsibility for Communities and Education, while Mayor Preston will oversee the areas of Adult Social Care and Public Health and Children's Safeguarding directly.
Mayor Preston said: "Thank you to Cllr Smiles for her ongoing professionalism and commitment to Middlesbrough.
"And I'd like to welcome Barrie Cooper who re-joins the team.
"I will bring other people onto the team in the coming weeks and in the meantime it's business as usual."
The Executive and the Mayor are responsible for key decisions the Council makes. By law the Executive must consist of the Mayor and at least two other members.
Details of the rearranged meeting of the Executive will be confirmed in due course.
