Johns, who has held the top PR job at Unilever for six years, will leave the Anglo-Dutch consumer goods giant at the end of May. It is not thought he has another job to go to. Before joining Unilever, Johns held senior comms posts at Sainsbury's and BT.
Weed's appointment points to the integration of marketing and comms functions at Unilever for the first time. It is thought that Weed, who reports to CEO Paul Polman, is planning to restructure both departments.
Unilever's corporate comms team comprises 20 staff in three departments - media relations, digital and internal communications.
Johns' departure is one of a number of changes to beset the Unilever management board since the arrival of CEO Polman early last year.
Weed was brought in to replace outgoing marketing chief Simon Clift, who stepped down earlier this year. Jim Lawrence, chief financial officer, left in February, and head of food, home and personal care Vindi Banga, one of Unilever's most seasoned executives, left in March.
In a high-profile interview with the Financial Times last month, former marketing chief Clift warned of a 'lost generation' of brand managers who do not understand the web and social networks.
In his final interview before retiring, Clift said he believed PR agencies were best placed to profit from the rise of Facebook and Twitter, as traditional advertising agencies struggle to adapt to the digital world.
Last week Unilever reported profits for the first three months of 2010 up 28 per cent to £906m on the same period last year. It is also suing the BNP for using its Marmite logo without permission in an ad.
Johns was included in PRWeek's 2010 Power Book. He listed his ideal activity for a year-long career break as 'sailing while reading the Patrick O'Brian naval series'.