After passing through the free school at Canterbury, he became clerk to his father, and Archbishop William Laud soon advanced him to be registrar of the ecclesiastical courts of the diocese.
Somner acquired great reputation as an antiquary, and he numbered among his friends and correspondents Archbishops Laud and James Ussher, Robert Cotton, William Dugdale, Roger Dodsworth, Symonds D'Ewes, Edward Bysshe, Thomas Fuller, and Elias Ashmole.
At the suggestion of Meric Casaubon he acquired a knowledge of Anglo-Saxon, and then wrote Observations on the Laws of King Henry I, published by Roger Twysden in 1644, with a new glossary.
James Brome, under the title of A Treatise of the Roman Ports and Forts in Kent, with notes by Edmund Gibson, and a life of the author by White Kennett.
He composed, in reply to Jean Jacques Chifflet, a dissertation on Portus Iccius, the place where Julius Caesar embarked in his expeditions to Britain, and fixed it at Gessoriacum, now Boulogne-sur-Mer.
To William Dugdale and Roger Dodsworth's Monasticon Anglicanum he contributed materials relating to Canterbury and the religious houses in Kent, and he translated into Latin all the Anglo-Saxon documents, and many English records for the same work.