Sir John, 56, former chief executive of the investment bank Morgan Grenfell, will take over the pounds 120,000 a year post from the current Lonrho chairman, Sir John Leahy, after the company's annual meeting on 26 March.
Morgan Grenfell, now owned by Deutsche Bank, is Lonrho's financial adviser and Sir John remains chairman of Deutsche Morgan Grenfell Group, part of Deutsche's investment banking division. However, Sir John said there was no conflict of interest since his sole role at the bank now was to bring in new business.
"My position at Lonrho is an entirely personal one," said Sir John "It has nothing to do with Deutsche Morgan Grenfell. They are Lonrho's financial advisers and I would hope and anticipate that will continue to be the case."
Sir John, who has several other directorships, said he would not be devoting a fixed proportion of his time to Lonrho but would be available to its chief executive Nick Morrell whenever necessary. His role will be to help in strategic development .
Mr Morrell is expected to outline his plans for the group in May or June after the European Commission has ruled on whether the South African mining group Anglo American can keep its 28 per cent stake in Lonrho.
Mr Bock agreed to sell his 18.8 per cent stake in Lonrho to Anglo for pounds 257m last October and step down as chief executive. Earlier this week Mr Bock severed all links with Lonrho by resigning as non-executive deputy chairman.
With the Metropole hotel chain sold to Stakis and the Princess hotel business up for auction, the plan is to demerge Lonrho's African trading businesses and other non-mining interests this summer. But these plans depend on whether Brussels blocks Anglo's investment.