In 715 he set out on a missionary expedition to Frisia, intending to be able to convert them by preaching to them in their own language, his own Anglo-Saxon language being similar to Frisian, but his efforts were frustrated by the war then being carried on between Charles Martel and Radbod, king of the Frisians.
Bonifiace again set out in 718, visited Rome, and was granted a commission by Pope Gregory II to reorganize the church in Germany and preach to pagans there. For five years he laboured in Thuringia, Hesse, and Frisia, and then returned to Rome to report his success. During this visit Pope Gregory II made him a bishop. He again set out for Germany, and, armed with full powers from the Pope, baptized thousands of the heathen, and dealt with the problems of many other Christians who had fallen out of contact with the regular hierarchy of the Catholic church.
After another visit to Rome in 738 he proceeded to Bavaria, and founded there the bishoprics of Salzburg, Regensburg, Freising and Passau. In 742, one of his chief disciples, Sturm, founded the abbey of Fulda not too far from Boniface's earlier missionary outpost at Fritzlar. Although Sturm was the founding abbot of Fulda, Boniface was very involved in the foundation. The initial grant for the abbey was signed by Carloman, the son of Charles Martel. The support of the Mayors of the Palace and later, the early Pippinid and Carolingian rulers, was important to Boniface's success. Boniface balanced this support and attempted to maintain some independence, however, by attaining the support of the papacy and of the Agilolfing rulers of Bavaria.
After returning from his mission in Bavaria, Boniface resumed his labours in Germany, where he founded the dioceses of Würzburg, Erfurt and Buraburg. By appointing his own followers as bishops, he was able to retain some independence from the Carolingian rulers. He also organised provincial synods in the Frankish Church, and maintained a sometimes turbulent relationship with the king of the Franks, Pepin, whom he may have crowned at Soissons in 751. Boniface had been created a bishop by Gregory II, and after the deposition of the bishop of Mainz in 745, Boniface was granted the metropolitan see.
He had never relinquished his hope of converting the Frisians, and in 755 he set out with a small retinue for Frisia. He baptized a great number, and summoned a general meeting for confirmation at a place not far from Dokkum, between Franeker and Groningen. Instead of his converts, however, there appeared a mob of armed pagans, who fell upon the aged archbishop and slew him. His remains were finally deposited in the abbey of Fulda.