John Gwyn Jeffreys was born on January 18, 1809, in Swansea, Wales.

He was a barrister, conchologist, malacologist, and author. He was educated in Swansea at the Bishop Gore School. In 1826, he became an apprentice to one of the principal solicitors of Swansea, before going to London in 1838. In 1856, he was called to the bar in London but his greater passion was for conchology. He was not satisfied simply to form a collection but was interested in all aspects of the biology of mollusks. In 1840, he became a Fellow of the Royal Society. In 1866, Jeffreys retired from the law and continued a series of dredging operations he started in 1861 aboard the yacht Osprey. Accompanied by other specialists in marine life, he dredged the seas around the Shetland Islands, the west of Scotland, the English Channel, the Irish Sea, and Norway. He participated in several deep-sea expeditions as a scientific leader, e.g., the Porcupine expeditions (1869, 1870), the Valorous expedition to Greenland in (1875), and the French Travailleur expedition (1880). He served as Justice of the Peace for Glamorgan, Brecon, and Hertfordshire, and was appointed High Sheriff of the latter in 1877. He was Treasurer of the Linnean Society of London and the Geological Society of London for many years. He was also a member of the British Association for the Advancement of Science. He was awarded an honorary degree of LL.D. from St. Andrews University. Jeffreys was the author of a number of books and articles on conchology and the mechanics of sea dredging, e.g., "British Conchology, or an Account of the Mollusca Which Now Inhabit the British Isles and the Surrounding seas" (5 vols., 1862-1865).

In 1840, he married Anne Morely Nevill (1815–1881). He died on January 21, 1885, in Kensington, Middlesex, England.