Most of his early adventures can be found in "The Boyhood Deeds of Fionn." He was the son of Cumhall, who was the leader of the Fianna, and Muirne, who was the daughter of the Druid Tadg mac Nuadat. It was prophesied that if Cumhall ever married, he would be killed in his next battle. He fell in love with Muirne, and even knowing of the prophecy, he wanted her. Cumhall stole Muirne after her father refused his offer of marriage for her. This caused Tadg to appeal to the High King, Conn of the Hundred Battles, who outlawed Cumhall. A great battle was then fought between Conn and Cumhall, later known as The Battle of Cnucha, and Cumhall was killed by Goll mac Morna who assumed leadership of the Fianna.

Muirne was already pregnant however, and thus her father rejected her. He ordered his people to burn her, but Conn would not allow this. He put her under the protection of Fiacal mac Conchinn, whose wife, Bodhmall the druidess, was Cumhall's sister.

After Fionn was born, Muirne left him in the care of Bodhmall and the warrior woman Liath Luachra. He was brought up in secret in the forest of Sliabh Bladma, and taught the arts of war and hunting. As he grew, he entered the service of several local kings, hiding his identity. When the kings recognized him as Cumhall's son, they told him to leave their lands, fearing his enemies would find him and they would not be able to protect him.

As a young man, Fionn met the druid and poet Finn Eces (also known as Finnegas), near the river Boyne and studied under him. Finn Eces had spent several years trying to catch the Salmon of Knowledge. It was said that whoever ate the salmon would gain all the knowledge in the world. Eventually he caught it, and told Fionn to cook it for him. While cooking, Fionn burned his thumb. Instinctively, he stuck it in his mouth to cool it, and swallowed a piece of the salmon's skin. And thus he gained the salmon's wisdom. This showed him how to gain revenge on Goll, and in subsequent tales he was able to gain more knowledge from the salmon by chewing his thumb.

Once a year, for 23 years at Samhain, the fire-breathing fairy Aillen would lull the men of Tara to sleep with his music. Then he would burn the place to the ground. Goll and the Fianna were powerless to prevent it. Fionn came to Tara with his father's crane-skin bag of magical weapons. Keeping himself awake by using the point of his own spear, he then killed Aillen with the same spear. His heritage was then recognized and he was given command of the Fianna. Goll willingly stepped aside for him and became his loyal follower. Though in many stories after this, their alliance is uneasy and feuds do occur. Fionn demanded compensation for his father's death, threatening war or single combat if refused. And so Tadg offered him his home, the Hill of Alan, and Fionn accepted.

Fionn met his most famous wife, Sadbh, when he was out hunting. A druid, Fear Doirich, had turned her into a deer. Fionn had 2 hounds who had once been human themselves, Bran and Sceolan, they recognized her as like themselves and Fionn spared her. She transformed back into a beautiful woman, married Fionn, and was soon pregnant. Then Fear Doirich returned. He turned her back into a deer and she vanished. 7 years later, Fionn found his son Oisin, who later became one of the greatest of his Fianna, but of Sadbh he never found a trace.