The asset manager, which invests funds on behalf of individuals and institutions, is to provide £113 million in finance for the development after working with ScottishPower and its development partner Helical Bar.
Work on the building, which will be able to accommodate up to 1900 staff, will begin later this year. The office is expected to be ready for staff in late 2015.
London-based M&G has acquired the land on India Street and St Vincent Street on a freehold basis, and will initially lease the property to ScottishPower for 25 years with annual reviews linked to inflation.
The power company, which is part of the Iberdrola Group, will be responsible for all outgoings, maintenance and repairs.
M&G has provided the capital for the development under its £1.53bn M&G secured property income fund, which invests in and develops real estate such as supermarkets, hotels and offices.
The assets are leased to tenants on a long-term basis, to provide pension fund investors with returns linked to inflation. So far, the fund has provided development finance of up to £400m.