Jacobi takes over Shell's top UK PR post on Monday from de Beer, who is retiring after 20 years with the Anglo-Dutch oil giant.
She joins from US investment house Lehman Brothers, where she held a number of senior responsibilities as chief brand strategist and global head of marketing and corporate relations, based in New York.
Lehman Brothers was unavailable for comment on Jacobi's replacement as PRWeek went to press, due to the chaos caused by the terrorist attack on the US this week.
On her role at Shell, Jacobi said she looked forward to 'enhancing the reputation' of the company.
She will report to Shell director of planning, environment and external affairs Malcolm Brinded and work closely with chairman of the committee of managing directors, Philip Watts.
Brinded said: 'Mary Jo brings an impressive track record in international corporate and public affairs and brand building.'
Prior to Lehman, Jacobi worked as an adviser to the board at financial services giant HSBC and head of the bank's group marketing and communications department in London.
Before moving into corporate PR, she was a senior adviser to US presidents Ronald Reagan and George Bush.