He is the Lord of Horn Hill and the head of House Tarly. He forced his son Samwell Tarly to join the Night's Watch under the threat of death.
Randyll Tarly is the Lord of Horn Hill and the head of House Tarly, a vassal family of House Tyrell of Highgarden. He is a fierce, ruthless warrior and a highly effective general. He is the father of Samwell Tarly and a younger son, Dickon, and at least one daughter Talla.
Along with his Liege Lord Mace Tyrell, Randyll fought on the Targaryen side in Robert's Rebellion. He commanded the royalist victory at the Battle of Ashford - the only defeat that Robert Baratheon ever suffered in the entire war.
Samwell recounts to Jon Snow that even though he was his father's eldest son and heir, he was disgusted with Samwell's "womanly" interest in books and learning and disdain for martial valor and combat. Soon before Sam turned eighteen, his father took him aside and bluntly told him that he was unworthy to inherit the Tarly lands and titles. Randyll ordered Sam to take the black and join the Night's Watch. Randyll told his son that if he did not, he would take him out on a hunt in the woods and kill him in such a way that it looked like an accident. Terrified, Sam did as he was bid and left Horn Hill for the Wall.
At Stannis Baratheon's war council, Davos Seaworth names Randyll Tarly amongst the many lords that have declared for Renly Baratheon.
Samwell mentions his father by name when explaining the concept of family names to Gilly. When she points out that Randyll is a handsome name, he asks her to please not give it to her newborn son.
Sam quips to Gilly that his father is not a very "literary man". Later, surprised that Samwell is Randyll's son, Stannis Baratheon recounts to Sam how Lord Randyll defeated Robert Baratheon in the Battle of Ashford — the only battle Robert ever lost, for which King Stannis considers Randyll a fine soldier.
Since the Citadel only allows men within its walls, Samwell decides to send Gilly and her baby Sam to live with his family at Horn Hill. While Sam admits that he does not get on well with his father Randyll, he reassures Gilly that his mother and sister are nice.
When Sam and Gilly arrive at Horn Hill, Randyll assumes she is a prostitute from Mole's Town, something that he manages to come to terms with. She is dressed in southern clothes, and she, Sam, Randyll, Dickon, Melessa, and Talla all have dinner together. At the table, Randyll repeatedly berates Sam: he insults his weight, tells him the Night's Watch failed to make a man out of him, and says he will never be a great warrior, or inherit Heartsbane, the Tarly family's ancestral Valyrian steel sword. Gilly rushes to defend him, telling Randyll that Sam killed a White Walker on his way down to Castle Black. By saying this, she lets slip that she is in fact a wildling, a group of people who Randyll apparently considers to be subhuman. He banishes Sam from Horn Hill, but allows Gilly and her baby to remain. However, Sam takes them both with him when he leaves, along with Heartsbane itself.
A legendary soldier, Randyll Tarly has no patience for his son Samwell's studious nature. He forced Sam to join the Night's Watch, making his second son Dickon heir to Horn Hill. When Samwell returned to his family's castle with Gilly, Randyll was disgusted to learn she was a wildling and banished Sam for good.