His father, Major J. S. Henderson, was half Scottish and half of the Irish family of Newenham: his mother was a Carus-Wilson from Westmorland. Both, however, were born and bred in Cornwall, and a portion of Cornish ancestry came to him through his mother's mother, one of the Willyamses of Carnanton in Mawgan-in-Pydar. He was at Wellington College for a short time but left on account of ill-health. For this reason he was frequently sent home from school for rest, and spent a large amount of his time walking over Cornwall and studying Cornish monuments and history. He collected a large number of documents from all over the county. Henderson went to New College, Oxford and took his degree with first-class honours in modern history in 1922. He was a lecturer at University College, Exeter, and afterwards at Corpus Christi College, Oxford, where he was elected to an official fellowship as tutor in modern history in 1929. He had settled down at Oxford, and was showing great promise as a teacher and lecturer. In 1928 Henderson published a book on Cornish bridges in collaboration with Henry Coates. Whenever he was able he would return to Cornwall and continue his historical research which in the early years was concerned very largely with the four western hundreds but finally he planned a parochial history of the whole county on a grand scale.
Henderson’s publications included Cornwall; A Guide in collaboration with J. C. Tregarthen, in 1925; three books on Cornish churches; and another on Cornish coasts, moors, and valleys with notes on antiquities. In 1928 he was made a Bard of the Cornish Gorseth at Boscawen-Un, taking the bardic nameMap Hendra. His collection of documents is held at the Courtney Library of the Royal Institution of Cornwall in Truro. The collection includes 16,000 ancient documents, many hundreds of transcripts in Henderson's hand, and his own writings either in published form or in manuscript. After completing his book on Cornish bridges, Henderson prepared notes for a similar book on the bridges of Devon. After his death the civil engineer Edwyn Jervoise completed the book and it was published in 1938.
Essays in Cornish History edited by A. L. Rowse and M. I. Henderson 1935 - Contents include: essays on Truro, the origin of towns, Fowey, Lostwithiel, Restormel Castle, Mitchell, Luxulyan, Helston, St Ives, the Deanery of Buryan, the Hundreds of Pydar and Powder, Twelve Men's Moor, Black-more, woodlands, and shorter pieces
Some Notes on the Parish of Goran, otherwise St. Goronus 1936
A History of the Parish of Constantine in Cornwall; edited by the Rev G. H. Doble. 1937
A History of the Parish and Church of Saint Euny-Lelant with Gilbert Hunter Doble and R. Morton Nance, and a description of the Church by M. H. N. C. Atchley. 1939
A History of the Parish of Crowan... with explanations of place-names by R. Morton Nance, 1939