The Attempted Murder
Through most of his combat in the Battle of the Blackwater, Tyrion Lannister was accompanied closely by 2-3 other figures: his squire Podrick Payne, Ser Balon Swann, and Ser Mandon Moore. While fighting on the "Bridge of Ships," Moore makes a brutal attempt on Tyrion's life only to be knocked into the water by Pod where he drowns.
[Charles Clifford, CC0, via Wikimedia Commons]
The Search for a Man without a Past
After awakening to discover his nose being 2/3 shorter than it was before he woke up, Tyrion asked "Ser Bronn" investigate Moore and his past. Bronn had littlle luck, finding that he had few to no friends and very little record of any activities outside his guard duties. Tyrion remembers how Jaime had told him that Mandon Moore was the most dangerous of any of the Kingsguard, because he never made any of his intentions known. He was not loved by the smallfolk, nor close to any of his brothers on the kingsguard.
Why Would a Nobody Try to Kill Tyrion?
[Unknown author, Public domain, via Wikimedia Commons]
How would a man known by apparently no one in King's Landing earn a spot on the Kingsguard?
To be honest, Mandon Moore's past is not entirely secret, nor is he really a "nobody." As Jaime recognized, he was a dangerous and secretive man with strong martial skill. However, we do know something about his family: House Moore.
The Moores are a noble family from the Vale of Arryn. According to GRRM's semi-canon The Sons of The Dragon novella, the Moore family has a history in the Kingsguard, serving under Maegor the Cruel.
"Lord" Petyr has an established history of having secret arrangements where armed men work for him, including infiltration of the Kingsguard -- the Kettleblacks, for example. Littlefinger came to court after his appointment as head of customs at Gulltown, where his financial wizardry impressed Jon Arryn, whose wife Lysa had suggested the appointment. While Jon Arryn was Hand of the King, Robert Baratheon brought Littlefinger to court, within three years rising to Master of Coin.
While we don't know when Ser Mandon came to King's Landing, we know he wore a white cloak when the story begins. It's likely that Ser Mandon became a member of the Kingsguard at the suggestion of Littlefinger, whose record of financial performance had (rightly) impressed the Hand and the King.
Alongside his official duties, Littlefinger also grew sources of income and wealth -- his brothels being the best known. Brothels were also a source of intelligence for Littlefinger, allowing him to listen to the causal talk of his patrons. As we learn when Catelyn first enters King's Landing, Lord Petyr had staff monitoring the gates of the city as well (though he probably expected Catelyn to arrive after having Lysa lie to her about who had killed Jon Arryn!).
As a man who made use of secret arrangements with armed men, it's very reasonable to believe that Baelish was the patron who brought Ser Mandon to King's Landing and who held him in debt, whether tangibly or not.
