McDonald, who in January moved to T Rowe from Citigate Dewe Rogerson, said 'it was time for a change', adding that he remained on 'good terms' with GA.
The former CDR head of investment said in January that T Rowe was interested in Canada and the Baltic states, and might seek external PR support to promote new product lines. T Rowe manages £155bn for institutional and private investors.
In Q3 last year, T Rowe spent £25.5m on promotion, according to Nielsen Monitor-Plus, compared with Fidelity Investments' £23.9m.
Fund managers have increased their promotional spend in recent years as equity markets recover.