Merrett served as a page and squire for Lord Sumner Crakehall at Crakehall. Jaime Lannister also served at Crakehall, but didn't like Merrett due to his habit of bullying the other squires. When Merrett tried to bully Jaime, it is implied Jaime turned it back at him.
Merrett caught a pox from a camp follower and was later captured by Wenda the White Fawn and branded. In his next fight he was hit on the head by a mace, leaving him insensible for a fortnight. As a result he suffers extreme pain whenever he hits his head and has turned to drinking to cope with it. His marriage to Mariya Darry turned out badly, as the Darrys supported the Targaryens during Robert's Rebellion and lost half their land and much of their wealth.
Merrett is not well-regarded at the Twins, considering his wife a shrew, despised by his father, and with a reputation as the biggest drunk at the Twins. He is known as Merrett Muttonhead.
When Robb Stark approaches the Twins, as part of the agreement for the Freys to join him he has to agree to marry a Frey and it is agreed Merrett's son Little Walder Frey and another of Walder Frey's grandsons Big Walder Frey will be fostered at Winterfell.
Robb after deflowering Jeyne Westerling marries her to preserve her honour. This and the Lannister victory over Stannis Baratheon due to an alliance with Mace Tyrell leads the Freys and Boltons to make an agreement against Robb. Roose Bolton, Lord of the Dreadfort, is offered the weight in silver of a Frey bride, so he marries Merrett's second daughter "Fat" Walda Frey. Merrett thinks this will earn him his father's praise, but is criticised for not curbing his daughter's gluttony and thus making Walder pay a larger dowry then necessary.
The Freys tell Robb they will re-join him if his uncle Edmure Tully, Lord of Riverrun, marries Roslin Frey, one of Merrett's half-sisters. Lothar Frey organises the Red Wedding, telling his half-brother Merrett to drink Lord Jon Umber "the Greatjon" insensible. However the Greatjon's outdrinks Merrett Frey, Petyr "Pimple" Frey (one of Walder Frey's great-grandsons) and Whalen Frey, leaving Merrett and Petyr unconscious. When the massacre starts, the Greatjon snatches a sword from the first man to attack him, breaking the man's arm. It takes eight men to put him in chains, and he kills one, wounds two, and bites of Ser Leslyn Haigh's ear. However Robb Stark, his mother Catelyn, and most of their forces are killed. Catelyn's body is stripped and tossed into the river as a mockery of Tully funeral customs. As part of the agreement with the Lannisters, Merrett's eldest daughter, the notoriously promiscuous Amerei Frey, will marry Tywin's nephew Lancel Lannister, the newly made Lord of Darry, which will assist his claim to the Darry lands.
Later Petyr Frey is captured by the Brotherhood without Banners and a hundred dragon ransom is demanded. Merrett volunteers to deliver the ransom. Walder Frey is 91 and failing. His eldest son Ser Stevron Frey died of a wound, meaning the heir is Stevron's eldest son Ryman Frey, a greedy fool. Merrett considers Ryman's two eldest sons, Edwyn Frey and Black Walder Frey, even worse, as they both aspire to succeed to the Twins and intend to throw most of their relatives out. Merrett will be unable to make an existence on his own, so hopes to win the favour of the main Frey branch by ransoming Ryman's youngest son Petyr.
However when Merrett arrives at Oldstones he finds Petyr has already been hanged by the Brotherhood. After they take the ransom the Brotherhood prepare to hang Merrett for his part in the Red Wedding. He tries telling them they'll get a bigger ransom for him, but they say his father won't fall for the same trick twice. Merrett claims that the Freys were justified as Robb dishonoured them. He tells them Lord Beric Dondarrion always gives a man a trial and they can't prove anything against him, all he did was drink. However the Freys reveal the resurrected Catelyn Stark, now Lady Stoneheart, still with the gash in her throat from Merrett's half-brother Raymund Frey. They ask her if Merrett was part of it, and when she nods Merrett is hanged.
The Freys search for the outlaws but are unable to find them, though Black Walder finds out they included a hooded woman, believed to be their leader.
Later Merrett Frey's son Little Walder is murdered at Winterfell, possibly by his cousin Big Walder.