Lyman's exact age is unknown, but he appears to have been born some time around 470 AC. He was raised on a farm with a host of other boys, an origin which he jealously keeps. He was generally unloved as a child and falsely considered it to be because of his superior intellect.
Eventually, Lyman came into the service of the Lannisters of Lannisport, working for them for many years until he was brought into the service of House Lannister of Casterly Rock by Lord Loren Lannister, where he oversaw the keep's finances.
After the Hand's death at the beginning of the third era Lyman began truly working for the King in earnest.
Lyman returned to King's Landing with the news that the Iron Bank had reopened its coffers to the Iron Throne. A seemingly empty gesture, as the war had already been won.
Damon told Lyman of his plan to promote him to Master of Coin, a position on the Small Council, and set him to deciphering the accounts left behind by his predecessor, Lord Orys Connington. Lyman returned to Damon with the news that the old Master of Coin, Baeron Pyke, working under King Harys Baratheon had been skimming gold from the crown's coffers, netting himself nearly 800,000 gold dragons before his 'disappearance' during the Sack of King's Landing. There was no way of tracking down the old Master of Coin if he survived the sack, as Lyman assumed, but the new Master of Coin was able to name six men who he claimed took part in Baeron's underhanded dealings. The Queen and the King gave Lyman the order to arrest them, the men being; Manfryd of Myr, a wealthy merchant who owned three wharf houses along the harbor. Jate and Jasper, brothel owners along the Street of Silks. Hibald, a man with fingers in half a dozen enterprises throughout King's Landing. Hobber of the Street of Looms. And Grunt, captain of the Quicksilver, a trading galley hailing from Braavos.
With commander Titus and six gold cloaks, Lyman arrested Manfryd of Myr. It seemed to be revealed that Lyman had not named the six men for traitorous actions, but because they were infringing on his own investments within the city.
King Damon Lannister set Lyman upon the task of arranging the repairs to the Kingsroad and brought the Master of Coin along with him upon his inspection of it. During this inspection, Lyman and the Sergeant of Arms for the Red Keep, Benfred Tanner, began antagonizing each-other.
"Upjumped coin counter... He claims he’s from Lannisport, but you know they say he's from Oxcross, really." - Harlan Lannett
"It hardly needed saying that face to face he was an exceedingly brilliant conversationalist, but Lyman had never felt quite at ease in large groups. They reminded him unpleasantly of his boyhood dinners at the farmhouse, where the other children would throw bits of food at him and prod him in the ribs until he cried." - Thoughts of Lyman
“A useful man, who clamoured over the back of friend and foe alike for position.” - Loren Lannister