Lord Petyr Baelish, sometimes called Littlefinger, served as Master of Coin on King Robert I Baratheon's small council and briefly on King Joffrey's small council. Petyr wore a mockingbird as his personal crest instead of House Baelish's sigil, a titan's head.
Events Before the War of the Five Kings
When Catelyn Tully arrives in King's Landing to tell her husband, Lord Eddard Stark, about the attempt on their son Bran's life, Petyr, who learned of her arrival despite her efforts to remain unnoticed, intercepts her and stashes her away in one of his brothels. Catelyn, justifying her trust in Baelish from their time together in Riverrun, shows him the Valyrian steel dagger used in the attempt by a catspaw. Petyr says that he recognizes the dagger, claiming that it was once his but that he lost it in a wager to Tyrion Lannister. This lie - Petyr in truth lost the dagger to King Robert I Baratheon - leads to Catelyn's eventual arrest of Tyrion at the crossroads for the attack.
Petyr fetches Eddard to meet with his wife and is able to convince him, against his inclination, to make no move against the Lannisters until they have proof. He vows to assist Ned in finding Jon Arryn's killer, and during Ned's tenure as Hand of the King Petyr feigns friendship to him on behalf of his fondness for Catelyn. He superficially attempts to tutor Ned in the intrigues of the court and warns Ned not to trust anyone, including himself.
When King Robert I learns that the exiled Daenerys Targaryen is pregnant, Baelish agrees with Robert in advocating her assassination. Ned Stark resigns as Hand in protest of Robert's command, but before he can arrange to travel back to Winterfell, Petyr approaches him with the promise of assistance in his investigation into Jon Arryn's death. He leads Ned to a brothel where a young whore is raising one of Robert's bastard children. He is present when Ned is accosted by men led by Ser Jaime Lannister while returning from the city shortly thereafter.
War of the Five Kings
After King Robert's mortal wounding and his subsequent naming of Eddard as Regent and Lord Protector, Baelish advises Ned that despite their knowledge that Queen Cersei Lannister's children are illegitimate and that Stannis Baratheon is the legal heir, the wisest course of action is to ensure Joffrey's succession, a maneuver calculated to avoid all-out war and maximize their own power. Eddard refuses the suggestion and enlists Petyr's help in securing the allegiance of the city watch to ensure its loyalty when he moves against Cersei. Baelish agrees, but betrays Ned, bribing the City Watch to support the queen. This leads to Stark's arrest and eventual execution for treason.
After the majority of Eddard's followers in King's Landing are killed, Petyr takes Jeyne Poole into his custody.
As the war wages on, Petyr remains in King's Landing to act as Master of Coin on King Joffrey's small council. During which, he formulates a plan to climb his ladder of chaos. Upon the newly self-crowned King Renly Baratheon's victory over his brother, Stannis Baratheon, Petyr knew he had to act fast before it was too late. His plan was in motion to whisk Sansa Stark away from the Capitol and to the Vale.
During the chaos of the [battle], Petyr escaped with Sansa in tow. Upon their arrival at the Vale, Petyr and Lysa Arryn wed which greatly upset the Lord's Declarant. Upon the murder of Lysa Arryn by a bard, the Lord's Declarant demand that Petyr, now Lord Regent of the Vale, hand over Lord Robin Arryn to the Lord's Declarant. During the meeting, Lord Corbray reacts violently which gives Petyr the leverage to demand a trial period on his wardship over Robin.
After a few months, Robin dies of what the maester said was natural causes most likely from his mental illness. Harrold Hardyng, next in line for Lordship, takes the 'Arryn' name and is named Lord Protector of the Vale. Soon after, Harrold and Sansa were wed at Petyr's behest. The realm finally knew of the fate of Sansa Stark; Lady of the Vale.
Petyr attempted to sway Harrold to swear to King Renly which took far more effort than he though. What he did not know was that Sansa was pushing Harrold in the other direction; to King Robb and the North. Sansa, upset with Petyr and his attempt to 'play the game' with her husband, convinces Harrold to swear to Winterfell, bringing The Vale into the realm of the North.
Soon after Lord Harrold swore fealty to King Robb, Petyr Baelish was found murdered and stripped naked with multiple gashes, stabs and slices. To this day, it is unknown how Petyr ended up alone, meeting his early demise.