The son of William Edward Harvey Grylls, of Winterbourne Zelston House, Blandford, Dorset, a Brigadier in the 15th/19th The King's Royal Hussars of a family that can be traced back to 17th centuryCornwall, he was educated at the Royal Naval College, Dartmouth. His eyesight was not good enough for the Navy, so he joined the Royal Marines, and saw active service, leaving in 1955, and studying Spanish at the University of Madrid. He turned his hand to business, setting up a wine importing firm called the 'Costa Brava Company'. His description of some of his products as "Spanish champagne" provoked the ire of both the makers of genuine champagne and its London importers; he was unsuccessfully prosecuted for trading under a false description, but they won a civil action against him for "passing off", i.e. misrepresenting his goods by using someone else's trademark. From 1959 he served as a councillor on St. Pancras Borough Council, and was elected to the Greater London Council for the Cities of London and Westminster from 1967–70.
For some years Grylls had acted as a consultant to the lobbying company run by Ian Greer at the heart of the Cash-for-Questions inquiry. The Parliamentary Commissioner for Standards, Sir Gordon Downey, stated that Grylls had "seriously misled" the select committee on members' interests in 1990 by understating the number of payments he had received for introducing clients to ministers and Grylls' conduct, said Sir Gordon, had fallen "below the standards the House is entitled to expect of its members". The Committee concluded that Grylls had "received payments from Mr Greer which were neither introduction commissions nor fees associated with the Unitary Tax Campaign. It is not possible to conclude that these payments originated from Mr Al Fayed, although Sir Michael actively participated in the Greer lobbying operation. Sir Michael deliberately misled the Select Committee on Members' Interests in 1990 by seriously understating the number of commission payments he had received; and by omitting to inform them of other fees received from Mr Greer. Sir Michael persistently failed to declare his interests in dealings with Ministers and officials over the House of Fraser. Sir Michael's action in taking a commission payment for introducing a constituent to Mr Greer was unacceptable. There is insufficient evidence to show that Sir Michael solicited business for Mr Greer in expectation of commission payments."
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Grylls was a member of the Royal Yacht Squadron until his death in 2001; his widow remains an honorary member. In Piers Morgan's Life Stories, Bear Grylls said that his father had a pace-maker fitted but suddenly died of a heart attack two days later while recovering at home.