Born earthside and commissioned into the UK element of the European Union's armed forces, Stanley Jackson served with distinction for years in the Royal Artillery, achieving the rank of Major, and distinguishing himself as a battery commander in the Afghan DMZ and in the Third Falklands War. But tales of space and the wonders outside Earth's atmosphere struck a chord in Jackson, and he resigned his commission, gambling that the Alliance would take him on as an officer of Marines.
The gamble failed, unfortunately, and Jackson was disqualified on grounds of age. Hurt, and with no desire to return home with his tale between his legs, Jackson was soon picked up by the turian Colonel Albrus Obsidio, who wanted a human's perspective on gunnery matters on the staff of his freelance artillery unit.
Over time, Jackson carved out a niche for himself in the unit, teaching the turian-formed outfit's staff the human perspective on the art of gunnery. Eventually, he earned himself the billet of Colonel Obsidio's aide-de-camp, and became one of the grizzled turian officer's most trusted confidantes.
Colonel Obsidio was killed during the Siege of Salookameye, his last words making Jackson the unit commander, with one last exhortion to "keep the guns roaring." That night, the unit's men renamed themselves in light of their new commander: Jackson's Brigade. Jackson promoted himself accordingly.
Since then, Stanley Jackson and his eponymous unit have distinguished themselves as one of the more proficient and professional mercenary bodies in space. The Brigadier runs the unit with spit-and-polish military discipline, and it very much shows, his men conducting themselves very much like professional soldiers. A unified rank structure and chain of command lend the Brigade more unity of purpose on the battlefield than some other mercenary units. Though they have taken contracts for some unsavory groups such as the Blue Suns, Jackson takes special care to make sure his men do not compromise their proffesionalism or honour as soldiers.
Personally, the Brigadier is an engimatic sort. He counts amongst his confidantes his grim turian Regimental Sergeant Major, Kadruk Bellix, and the Brigade's high-spirited Adjutant, Captain Mike Yates. Despite that, he rarely opens up to anyone. Rumors of an illicit romance between him and the Brigade's asari chief medical officer have been circulating amongst the gunners for ages, but remain unconfirmed.