Brookes replaces Paul Harris, the former Unite Students corporate affairs director, who left the role at Williams & Glyn in June after six months.
The news comes as rival Santander makes a formal offer to buy the circa 300 Williams & Glyn branches.
Williams & Glyn is made up of the parts of RBS and NatWest that the banking group was required to divest as a condition of its EU bailout during the financial crisis.
Although the Williams & Glyn branding is being rolled out across the branches and online this year, the split with current owner RBS will only take effect legally in 2017.
PRWeek reported in May that Brookes had stepped down from his role as MD at MHP, where he headed up the agency's financial services arm. Brookes had joined MHP from BNY Mellon, where he was latterly international head of marketing and corporate affairs.
Brookes told PRWeek today: "I am pleased to be joining the bank at such a key point in its divestment from RBS. In a world dominated by change, this is one of the most exciting roles in the industry and I am relishing the opportunity."