Service with ComStar
Both Karl and his sister Adrienne entered service with the Blessed Order in the early 30th Century. While his sister joined the Communications Branch, ultimately earning a position on the organization's ruling First Circuit and being selected as Primus Hollings York's chosen successor, Karl served in the organization's intelligence arm ROM during York's reduction and minimization of the branch in favor of his ComStar Guards and Militia. After the assassination of the publicly beloved York at the cornerstone laying ceremony for the refurbished Sandhurst Royal Military College in 2947, his sister Adrienne became Primus and among her first orders was to reverse the winding back of ROM and authorize expanded resources and funding which a grief stricken First Circuit quickly approved.  
She also dismissed Precentor ROM Gilbert, heavily wounded during the assassination of York, and selected her brother as his replacement. Whether the shadowy intelligence organization directly or indirectly caused the assassination of the openly hostile York, the increase of funding and elevation of Karl to head the branch helped ensure ROM unflinchingly supported her and she in turn referred to it as the core and essence of preserving ComStar. Karl would continue to serve as Precentor ROM for the remainder of her reign, even after she began to suffer from horrific nightmares and possible delusions of monstrous invaders from the Deep Periphery that triggered her creation of the Explorer Corps. 
Ten days after Yin Takami assumed the mantle of Primus following the death of his sister in 2979, Karl Sims attended a meeting of the First Circuit to inform the body of news of the Taurian Concordat's discovery of the derelict Black Lion-class SLS Tripitz. Sims also revealed to the First Circuit the Taurian's belief they could restore it to function as well as indications the Capellan Confederation's Maskirovka had also learned of the discovery and planned to stage their own recovery mission. The threat to ComStar's carefully cultivated technological superiority prompted Primus Takami to order the First Division of the Com Guards to destroy the derelict while shrouding their identity. After later providing yet more evidence of the Great Houses efforts to try and recover some of the LosTech from the Star League era, Sims would also oversee Primus Takami's call for what became Operation Holy Shroud II, a wide-ranging wave of assassinations of the Inner Sphere's greatest scientific and engineering minds. Unable to match the full extent of Janice Laidlaw's original Holy Shroud, Sim's ROM agents only had a number of small scale successes, though did ensure as before that none suspected ComStar's involvement in the assassinations and "accidents". 
Karl Sims would remain Precentor ROM through the remainder of Yin Takami's reign and that of his successor Allen Rusenstein, fulfilling Primus Rusenstein's so-called "Jolly Roger Plan" to support Periphery Bandit Kings to fan the flames of conflict. His reputation tarnished after the plan spiralled out of ComStar control, Sim's hold on the post of Precentor ROM would suffer a mortal blow with the shock arrival of the Wolf's Dragoons mercenary unit in 3005. With ComStar just as much in the dark as to origins of the mysterious mercenaries, a furious First Circuit demanded immediate results from the aging Sims. Unfortunately despite his best efforts, Sim's operatives were unable to pierce the Dragoons' security, with two agents dying in the attempt. Equally desperate for results to mollify a hostile First Circuit, Primus Rusenstein hastily removed Karl Sims and appointed Vesar Kristofur as the new Precentor ROM with the express mission of learning the truth about the mysterious Dragoons. 
- ComStar, p. 43 "History - Age of Enlightenment"
- ComStar, p. 44 History - Calm Before the Storm"
- ComStar, pp. 46-47 "History - Calm Before the Storm - The Tripitz Affair"
- ComStar, p. 49 "History of ComStar - Calm Before the Storm - The Jolly Roger Affair"
- ComStar, p. 50 "History - Calm Before the Storm - The Mysterious Dragoons"