Mal was brought up on a ranch on the planet Shadow. Raised by his mother and "about 40 hands", Mal apparently received a fairly decent education whilst growing up. Though Mal usually seems more practical than intellectual, he occasionally surprises his friends by displaying familiarity in a wide range of literature varying from the works of Shan Yu to poems by Samuel Taylor Coleridge.
Mal volunteered for the Independents army during the Unification War against the Alliance, gaining the rank of sergeant during that time. His loyal second-in-command Zoe was by his side for most of the war, surviving many dangerous conflicts with him. The show mentions three such battles, including the Battle of Du-Khang in 2510, and a long winter campaign in New Kashmir where he commanded a platoon. Mal was also involved in the ground campaign during the Battle of Sturges, which according to Badger was the "bloodiest and shortest battle in all the war", although Mal considers it a distant second. Mal fought in many more battles, but the turning point for him and the Independents came with their physical and emotional defeat at the Battle of Serenity Valley on the planet Hera.
After the war, Mal acquired his own ship, an old (broken) 03-K64 Firefly-class transport whose previous owner was a man named Captain Harbatkin (Mal never got around to changing the registration papers). He hired his crew, consisting of Zoe, Jayne, Kaylee, and Wash. He leased out one of his shuttles to a registered Companion named Inara, and began his career as a captain-for-hire. He often transports goods under the radar, people, and just about anything else that doesn't register or requires the attention of the Alliance. On one of these travels he picked up three more people who would join the crew, Shepherd Book, Dr. Simon Tam, and his surgically altered, psychotic, and somewhat psychic sister River Tam. Hijinx ensued cross the 'verse.
Mal's latest adventure ended on Mr. Universes station uploading the message found on the planet Miranda.
DISCLAIMER: I'm not Nathan Fillion, or know Fillion, Mal is property of Joss Whedon. Please don't sue me, Joss!