The appointments were announced at ICCO's global summit in Helsinki.
Mitchell had been elected vice president in 2015, serving under Maxim Behar during the Bulgarian's two-year term. As per ICCO rules, she now becomes president until 2019.
Following a vote of the ICCO executive committee - comprising its five regional presidents, president-elect, treasurer and CEO Francis Ingham - Mantri was named vice president. He will then become president in 2019.
Mitchell's firm Mitchell Communications Group is among the 100 biggest comms agencies globally, and the 54th biggest in the US, according to the PRWeek Global Agency Business Report.
Mantri is CEO at Avian Media and the co-founder of Chase India, a public policy and regulatory affairs firm. He is also president of the Public Relations Consultants Association of India.
ICCO represents more than 2,500 PR agencies based in 55 countries.