Mr Walmsley, who left Enterprise nearly a year ago, will succeed Peter Elwes, who will become deputy chairman. "He's got broad and relevant experience," said Simon Flowers of NatWest Markets.
"I would say he's a good catch for Hardy."
The appointment ends months of speculation about where Mr Walmsley might surface. He was short-listed for the post of finance director at Ladbroke, the betting and leisure group, following the death of Jerry O'Mahoney in June. There were also rumours that he might join Lasmo, for which Enterprise made an unsuccessful bid last year.
Mr Walmsley, 47, spent nine years at Enterprise, a period that saw the explorer's market capitalisation grow from £384m to £2bn. The reasons for his departure from the oil group he helped to found were never made clear, though a difference over strategy with chairman and chief executive Graham Hearne was a theory that gained currency at the time. He left several months before Enterprise's abortive bid for Lasmo.
At Hardy, which was demerged from Trafalgar House in 1989, Mr Walmsley has said he will concentrate on core areas such as exploration. Mr Elwes, who interviewed Mr Walmlsey for his first job at Enterprise in 1984, confirmed this yesterday. "Our strategy is unchanged. John has been through our plans with me and, broadly speaking, he agrees with them."
Hardy has interests in the US, in the North Sea, Australia and Pakistan. Some analysts feel Mr Walmsley may choose to focus on just a few core areas. "It's quite difficult to keep that many balls in the air at once," one said.
There is also a belief that he may want to wean the company off regular rights issues, which it has used to fund its expansion.