Manwani is currently president, Asia Africa, Central and Eastern Europe and will take up the position of chief operating officer on 1 September.
He will be responsible for all markets in order to "drive speed-to-market" and accelerate the rollout of innovations across markets.
Mike Polk, Unilever's president of global foods, home and personal care, will leave the company after eight years in the post, effective from 18 July.
Polk will move to Newell & Rubbermaid, the global company that owns brands Sharpie and Papermate, to become chief executive and president.
Unilever's category organisation will expand its three divisions to four under the new changes, adding a "refreshment" division to the current personal care, home care and food line-up.
The refreshment category will include ice cream and beverages (housing brands such as Wall's and Lipton), led by Kevin Havelock, who will take up the role as president from his current position as executive vice president, ice-cream.
Dave Lewis, currently president, Americas, will take up the position of president, personal care, overseeing all work for skin, deodorants, oral and hair products, which include Dove and Lynx/Axe.
Antoine de Saint Affrique, currently executive vice president, skin, will become the president of Unilever's food category, which includes savoury, spreads and dressings and brands such as Knorr and Flora.
The home care category will be jointly run by Randy Quinn, current executive vice president laundry, and Sean Gogary, senior vice president household care.
Each category head will report directly to Unilever chief executive, Paul Polman, who said: "Unilever now has over half its turnover in the emerging markets, where, over the past 10 years, growth has been close to double digits.
"We have an opportunity to better support this footprint of the business, to keep our strong momentum, with a more globally aligned country and category organisation."
Unilever said that the reshuffle will "further support" its growth plans in emerging markets and fast-developing businesses, enabling the company to roll out bigger innovations more efficiently.
The company said the changes will be put into place during the third quarter of the year, aiming to be fully operational before the year ends.