Born Alyn of Hull in 115 AC on Driftmark, a year after his brother Addam, to Marilda the shipwright's daughter, who claimed both boys had been fathered upon her by Ser Laenor Velaryon, though Mushroom in his Testimony says it was Lord Corlys himself, kept quiet and far from court so as not to offend the Sea Snake's fiery wife. Archmaester Gyldayn allows that the matter is still disputed. The brothers grew up in their grandfather's shipyard, small and quick like their mother and silver-haired and purple-eyed like old Valyria, and served aboard her trading cog Mouse. When Prince Jacaerys called for dragonseeds to come to Dragonstone in the Sowing of the Seeds, Addam won Seasmoke, and Alyn went looking for the wild Grey Ghost and could not find him. He tried Sheepstealer instead. The dragon set his cloak alight in its own lair; Addam beat out the flames with his own cloak while Seasmoke drove the beast off, and Alyn walked away burned across the back and legs, scarred for the rest of his life and counting himself fortunate, for other men who had gone after Sheepstealer were eaten. Lord Corlys petitioned Queen Rhaenyra to strike the taint of bastardy from both brothers, and she legitimized them and named Addam heir to Driftmark.
At fifteen it was Alyn who commanded the Velaryon ships that kept King Aegon II from returning to King's Landing after the fall of Dragonstone, and Alyn who carried the king back to the capital aboard his mother's old Mouse once Queen Alicent had come to terms with Lord Corlys. His ships were the king's only defense at the Gullet, and after a short siege the servants of Dragonstone yielded the castle up to him. Addam died proving his loyalty at the Second Battle of Tumbleton, and so it was Alyn alone who took Driftmark when the Sea Snake died in 132 AC. He and Marilda bore Corlys home aboard Mermaid's Kiss and buried him at sea from the deck of his Sea Snake. His cousins Daemion and Daeron advanced claims of their own and were ruled against by the regents; two of the silent five, Ser Malentine and Ser Rhogar Velaryon, tried to murder him in Castle Driftmark, and one died in the attempt while the other took the black. Ser Tyland Lannister thought him too young for a regent's seat and passed him over for Lord Unwin Peake. Later that year Lady Baela Targaryen, the wilder of Prince Daemon's twin daughters, fled the Red Keep rather than be married off by those same regents, came to Driftmark, and confided in him, and they were wed in the sept at Dragonstone a fortnight after.
King Aegon III named him lord admiral in 133 AC, but Unwin Peake gave command of the rebuilt royal fleet to his uncle Gedmund and ordered Alyn to nursemaid him in the Stepstones. It was Alyn who broke the Braavosi there, sinking the Grand Defiance with his Queen Rhaenys like a great oaken fist, and he came away from that fight with the name Oakenfist and the style Hero of the Stepstones, a knighthood from Ser Marston Waters, and the office of master of ships. Peake sent him next against the Red Kraken in the hope of being rid of him, and that voyage around what Alyn called the arse of Westeros became the first of his six great voyages, made on six different ships he called his ladies where his grandfather had made nine upon one. He drank and wrestled and killed at Racallio Ryndoon's whim on Bloodstone, took the favor of Princess Aliandra Martell at Sunspear, found Dalton Greyjoy already dead by a salt wife's knife, and learned from Drazenko Rogare on the homeward leg that Prince Viserys had survived the Dance and lay in Lysene hands, then sailed to Lys and bought him back from Lysandro Rogare for a hundred thousand golden dragons. In 138 AC he brought Addam's bones home from Raventree Hall and set one word on the tomb, LOYAL. In 157 AC he broke the Planky Town for the Young Dragon's conquest of Dorne and swept halfway up the Greenblood while Daeron I fought through the Prince's Pass, and came back to subdue both again in 160 AC when the Dornishmen rose. His marriage to Baela was a stormy one, and the woman he loved was Princess Elaena Targaryen, who bore him the bastard twins Jon and Jeyne Waters. He vanished at sea sometime between 171 and 176 AC. A year after he was lost, Elaena, who had hoped to marry him, consented to wed another.