The only son of Jurat William Laurence De Gruchy and Augusta Chambers Smith, he was born in St Helier on 22 March 1867. In 1870 the family moved to Beau Coin, St Brelade, and then to Rochebois, St Aubin.
He was educated at J E Vibert's School, St Aubin, from 1877 to 1881, and then at Wellington College until 1884. He entered Gonville and Caius College, Cambridge, in 1885.
He had already begun to take his lifelong interest in birds. The Jersey bank failures of 1886 forced him to earn his living, and he entered the Bank of Rio de Janeiro, and in 1889 was sent from London to the Rio branch. In 1891 he moved to the office of Norton, Megaw and Co, English merchants in Rio, where he remained till 1908, when he took charge of their head office in London.
In 1914 he became a director of his firm, and finally chairman of the board, retiring in 1931 after 40 years service. While in London he was an active member of the Jersey Society there, writing two of its publications, The Normans in Sicily (1909) and The Settlement of Normandy with special reference to the Channel Islands (1911).
In 1909 he bought Noirmont Manor, and settled there in 1916. Its grounds afforded welcome opportunities for bird study. From 1915 to 1940 he wrote annual reports on ornithology for the Bulletins of the Société Jersiaise, and was president of the Société from 1932 to 1934.
He was a keen student of the island's history, and his interest in this led him to revive the old Seigneurial Court for the Noirmont tenants. He wrote erudite articles for the Bulletins on Entries relating to Jersey in the Great Exchequer Roll of Normandy, The Family of Walsh the Seigneurie of St Germain, The Perquages of Jersey, and similar historical subjects. A few weeks before his death he finished a book on Mediaeval Land Tenures in Jersey. He was one of the three editors of the Cartulaire des Iles Normandes.
The palaeolithic cave-dwelling at La Cotte stood on his property. He not only gave the Société permission to excavate, but presented most of the finds to the Société's Museum.
He first became prominent in Jersey public life by fighting an attempt to extend British income tax to residents born in the United Kingdom. In 1927 he became treasurer of St Brelade's Hospital, and in 1928 was elected Constable. In both capacities he was brought up against the problem of pauperization, and he determined to find a remedy, while he was in the States.
A heart attack, which occurred as he walked up Bulwark Hill, on his way home from a day at the parish hall, made him resign in January 1931, but his health improved, and he was elected Jurat in August of that year. He then specially interested himself in housing, social assurance, and old age pensions. Bills on the first two were successfully piloted by him through the States ; and much work was done on the Pensions scheme, though he did not live to see this become law.
As President of the Harbours Committee he took an active part in constructing the Airport. While President of the Defence Committee four important subjects were dealt with, aliens, militia, police, and air raid precautions. As President of the Committee for the Preservation of the Natural Beauties of the Island it gave him great satisfaction when the States took over a stretch of cliffs on the north coast.
In 1938, however, his political work came to an abrupt conclusion. The States auditor challenged the right of the Harbours Committee to provide on its own authority pensions for its employees and to grant an honorarium to the engineer, who had supervised the building of the Airport. In the first case the pensions had appeared for years in the pay-sheets approved by the Auditor. In the second the payment had appeared in the Treasurer's Accounts. In each case they had apparently escaped notice.
The President of the Finance Committee now carried a resolution in the States that the Harbours Committee had acted ultra vires. As this was virtually a vote of censure, de Gruchy resigned his Juratship, feeling that no useful purpose could be served by continuing his work with a hostile majority against him.
In June that year he became very ill, and henceforth was subject to periods of severe illness. He died on 27 November 1940. In 1916 he married Catherine May, daughter of H Campbell Miller, and left a son, Philip, and two daughters, Hope May and Elisabeth Noèmi.