A half-dozen charters in Oxford between 1129 and 1151 were witnessed by him, so he was definitely in the Oxford area during that time. He was ordained Bishop of St. Asaph by Archbishop Theobald of Bec in 1152, although he doesn't seem to have ever actually spent time at St. Asaph's because of the wars of Owain Gwynedd. He likely died by Christmas 1154, when he was succeeded by Bishop Richard.
His importance to the modern world was the time he spent writing, especially the Historia Regum Britanniae ("The History of the Kings of Britain"). Although he claimed it was a translation of an ancient book—a common boast of medical writers to give authenticity to their work, which was more important than claiming originality—it is a combination of the works of Bede, Gildas, the Historia Britonum, anecdotes from oral tradition, and his own powers of invention. Future writers like Henry of Huntingdon drew on it without question, and from Geoffrey's time until the 16th century it was accepted as accurate history. (To be fair to medieval historians, William of Newburgh (1136 - 1198) did declare that everything Geoffrey said about Vortigern and Arthur was made up.)
He starts his history with Brutus the Trojan, the great-grandson Æneas, founding (and giving his name to) Britain, and Corineus the Trojan founding (and giving his name to) Cornwall. One of his descendants, Leir, divides his kingdom between his three daughters (General, Regan, and Cordelia), giving a later Shakespeare fodder for one of his tragedies. Books Five and Six deal with Vortigern and Merlin, then Book Seven breaks up the history with a series of prophecies by Merlin, setting up not only the later chapters, but also events in Geoffrey's own time. Books Eight, Nine, and Ten tell the Arthurian story, ending with the return of the Saxons after Arthur's death.
Geoffrey's Historia was enormously popular, with about 200 extant manuscripts known as of the 20th century. His section on Arthur—and the Prophetiae Merlini ("Prophecies of Merlin") and the (attributed to him) poem Vita Merlini ("Life of Merlin")—have provided modern retellings of the Arthurian myth in story and cinema with plenty of dramatic details.
As mentioned above, there were historians like William of Newburgh who were more critical when it came to selecting their material and relating it to an audience. William, however, was not immune to relating stories whose interest for the audience was more important than his ability to confirm them. Medieval clickbait? Let's find out tomorrow.