L was orphaned during the 3rd Great War in Kirigakure as a child, and was found by a man named Quillish Wammy (also known as Watari) who took him in to a school for the "gifted" in Kirigakure at a young age after seeing L's skills in deduction and observation at a very young age; without these skills L would surely have been left to fend for himself or die like many other orphans.
During his time at this school he further developed and advanced these skills to perfection, he shone the brightest in this school even outclassing other classmates and fellow colleagues (Near and Mello to name a few) though also persecuting himself and staying away from social activity, a trait he would still possess in his later years.
L was later assigned a position at Kirigakures hunter-nin task force, he worked solving cases and mysteries, finding the whereabouts of several missing nin organizations and bringing to justice several high ranking dangerous criminals in such a short period of time. This gained him leverage to becoming Kiri's lead detective as every single case he took on was accomplished with success and precision.
Nowadays L still is Kiri's lead detective, though still very young in his early twenties, and he still enjoys taking cases head on, though of course he's been trained in combat as required by the hunter nin, and he still refuses to relax having dark circles under his eyes that are only surpassed by Gaara of the Sand's, he works with Watari the man who brought him in and he owns a small bakery in the heart of Kirigakure which is not only for a cover but to also satisfy his sweet tooth.