Note: Vague Citations have been included to make it easier to find information in the source material.
Jarlaxle was born Jarlaxle Baenre of House Baenre, First House of Menzoberranzan. As the third born male (“thirdson”) he was to be sacrificed to Lloth, the Spider Queen, Goddess of Chaos, and She whom the drow of Menzoberranzan worship. Matron Mother Baenre did not realize that Lloth herself had blessed Jarlaxle and favored him, having named him as a mortal agent of chaos. The Matron of House Oblodra alone realized that Lloth would reject the sacrifice and put Matron Baenre out of Lloth's favor and so created a psionic energy shield around the baby to keep Matron Baenre's dagger from penetrating his flesh. When it became clear the sacrifice would not work, Baenre ordered the babe drowned in Donigarten (the local lake). As his elder brother Doquaio, the current secondboy of the House, picked him up all of the kinetic energy from the psionic shield transferred to him. Though he was barely a couple hours old (at the most) Jarlaxle killed his older brother because of the energy transfer and took his place as secondboy. Later in life, Lloth gave him these memories to show that she favored him. In spite of that, Jarlaxle still rejected her (Road of the Patriarch).
Not much is known about Jarlaxle's childhood. Presumably, he was raised in House Baenre and Triel was possibly his wean-mother (though at one point he claims to have never been a part of House Baenre) and as all nobles do, he attended Tier Breche. Jarlaxle attended Melee-Magthere, the fighting academy (Starless Night). As a fighter, he should have attended Tier Breche from age 20-30. The first nine years of his schooling would have been at Melee-Magthere, with the first 50 days spent in a brainwashing class where he would have learned all about the evils of the lesser races (any race that is not drow). After that, he would have rounded out his education with six months each at Sorcere (the academy for wizards) and the academy for clerics (future priestesses of Lloth), Arach-Tinilith (Homeland). At some point between attending Tier Breche (and some fans hypothesize that it was during this time) and before Homeland, Jarlaxle met and became friends with legendary hero Drizzt Do'Urden's father, Zaknafein Do'Urden. According to Jarlaxle, he and Zaknafein had common ground in that they were both thriving in the Underdark, a place that both males hated. They had a great respect for each other, and later he claims that this is why he did not let Artemis murder Drizzt, as a favor to his friend (The Silent Blade). Zaknafein had always thought Jarlaxle the better fighter, and it was Jarlaxle he went to in order to brag with fatherly pride about Drizzt's fighting skills (Servant of the Shard). It should be mentioned, however, that Jarlaxle did later betray Zaknafein, though he seems to regret it.
Sometime after he graduated from Tier Breche and about 200 years before Homeland, Jarlaxle left House Baenre to form his own group, the mercenary brand known as Bregan D'aerthe. This makes him one of the few males in Menzoberranzan ever to be expelled from his House and still be permitted to live (Road of the Patriarch). Bregan D'aerthe has been pivotal in many of the plot points relevant to the Legend of Drizzt books; thus, so has Jarlaxle. Jarlaxle first shows up in Exile, the second book in the Drizzt series. During this time, Drizzt has just spurned Lady Lloth and run away from home and his mother, Matron Malice Do'Urden, is anxious to catch him to regain Lloth's favor and avoid war upon her house. In order to wage war in a more covert manner, she hires Bregan D'aerthe to assassinate a noble member of House Hun'ett. Immediately after, soldiers of the enemy House attack House Do'Urden, with members of Bregan D'aerthe among them. Once Malice offers to pay him whatever he wants, Jarlaxle orders his soldiers to turn on those of House Hun'ett, striking them down and saving House Do'Urden. It is here that the beginnings of respect and friendship with Dinin, the elderboy of House Do'Urden (and therefore Drizzt's older brother), form as they watch the battle below together on the balcony of House Do'Urden.
Throughout the events of Exile, Jarlaxle serves as an informant and profiteer to every side, though once he has been forbidden (and most likely threatened) by Matron Baenre against speaking of something, he does not. When Malice asks him for information about other Houses moving against hers, Jarlaxle pointedly does not answer her question. This drives Dinin to seek him out on the streets to find out what is going on for his own sake. Jarlaxle gives him no information of value, however, which somehow seems to only confirm Dinin's concerns about the direction his House is headed. Shortly thereafter, Jarlaxle plays witness to Matron Baenre's final conversation before Zin-carla (the method used to chase down Drizzt, involves a zombie and angst) fails. The conversation takes place at House Baenre, and during this talk, Bregan D'aerthe slips in amongst the Baenre soldiers, just in case Zin-carla fails (which it does). Baenre and Bregan D'aerthe attack House Do'Urden, bringing it down. During the attack, Jarlaxle comes to Dinin, and accepts him into the band of houseless rogues along with Vierna, the sole surviving female of the House.
For a time, Jarlaxle and Bregan D'aerthe carry on as normal, playing Menzoberranzan's games of intrigue and pitting House against House for the sake of coin, gems, and entertainment. Twenty years after Drizzt has fled the City of Spiders, however, a great plan is put forth to go and capture him. The motivation behind this is to find some way to get into Mithril Hall and conquer the dwarves there (who are, coincidentally, Drizzt's friends). Triel Baenre (the Matron of Arach-Tinilith and therefore the highest ranking priestess after her mother, Matron Baenre) convinces Vierna that if she takes Bregan D'aerthe and goes to bring back Drizzt, that the Spider Queen will reward Vierna by putting her in Matron Baenre's place. Jarlaxle is surprised to learn of the plan at first, but as events unfold he finds himself more intrigued. This is also when he meets the assassin, Artemis Entreri, who hates Drizzt and will later become a valued companion to Jarlaxle. He is the one who makes contact with Artemis and uses him to lure Drizzt down into the caves below for capture. During this time he and Artemis seem to have a pretty good understanding of each other - even going so far as to like each other and seem to have a good deal in common.
After the final battle with Drizzt and his friends, Jarlaxle uses magic to flee to Menzoberranzan because Drizzt refuses to kill him and later returns for poor, dying Artemis. Taking Artemis back to Menzoberranzan, Jarlaxle keeps him as a member (though the lowest ranking, for Artemis is human) of Bregan D'aerthe, protects him, and even covers up a murder the human commits. He takes Artemis with him to important places, laying out the events that unfold during Drizzt's escape after the renegade has snuck back to the city and been captured, by covertly giving Artemis the passwords he needs to break into Gromph's office and take the spider mask to bust Drizzt out of House Baenre's dungeons. He is also the one who orders Cattie-brie to be taken alive, and does not permit any harm to come to her, making sure that Artemis has an ally in his escape plan. Other responsibilities given to Jarlaxle during this time include drawing up the plans for the invasion of Mithril Hall.
Drizzt, Entreri, and Cattie-brie escape Menzoberranzan, ruining the High Ritual that was to be Drizzt's death. A stalactite is knocked through the roof of House Baenre's great chapel thanks to them, and so Matron Baenre sends Jarlaxle out to discover whether or not the other Matrons are conspiring against her as a result since the destruction of the chapel could be construed as a sign that Lloth is displeased with House Baenre. However, Matron Baenre is not the only one interested. During this time, the only decent look at Jarlaxle's relationship with his elder brother is shown. Worried that there is a conspiracy, Gromph goes to Jarlaxle to question him further, concerned that his sisters might move against their mother to replace her (Starless Night).
Then everybody loses their magic when the Gods are forced to inhabit the Prime Material Plane (the mortal realm) during what is commonly known as the Time of Troubles. Jarlaxle's role in this is primarily as an informant and a giver of misinformation as he plays all the sides (primarily House Baenre vs House Baenre's children vs House Oblodra) against each other once again. However, in this situation, he finds himself rather uneasy about it all. Once the Time of Troubles has passed and House Oblodra has been eradicated for the trouble they cause, Jarlaxle finds himself serving as an adviser to Matron Baenre once again on the matter of taking Mithril Hall. He (very reluctantly) joins the group that ventures forth to take Mithril Hall when he is given no choice in the matter by his mother, bringing Bregan D'aerthe with him. When things begin to turn ugly and it becomes clear precisely how well the dwarves are fighting, Jarlaxle orders Bregan D'aerthe to stay out of the conflict, and avoid all fighting, acting instead as observers. The battle is lost and afterward Jarlaxle recruits Berg'inyon Baenre (the youngest Baenre) into Bregan D'aerthe and they retreat back to Menzoberranzan (Siege of Darkness).