The Free Worlds League was formed in 2271 by the Treaty of Marik, which united the Marik Commonwealth, the Grand Duchy of Oriente, and the Principality of Regulus. This was a loose organization of states, and they began to expand into the surrounding systems. In 2293, Juliano Marik annexed the Stewart Commonality, and was designated Captain-General. Since then, House Marik has held the post of Captain-General until the present day. Beginning in 2398, the Capellan Confederation declared war on the League in an attempt to annex Andurien and it's surrounding systems. This began the 150-year period known as the Age of War. During the Age of War, the Terran Hegemony developed the first BattleMechs. In 2556, Ian Cameron, leader of the Terran Hegemony, negotiated the Andurien Peace Treaty, ending the Age of War, and setting the foundations for the creation of the Star League. The Star League lasted for roughly 200 years, of which 150 were a time of unprecedented growth and discovery for all of mankind. After the Civil War against Stefan Amaris, the Usurper, Aleksandr Kerensky left the Inner Sphere with approximately 80 percent of the Star League Armed Forces.

Minoru Kurita and John Davion both seized the opportunity and declared themselves First Lord of the Star League shortly after, and Kenyon Marik followed suit in 2786, and the First Succession War began. The First, Second, and Third Succession Wars saw little change in the Inner Sphere, except for the tremendous loss of life and technology. Soon, the Ares Conventions were established, governing the conduct of warfare, and war became a way of life in the Inner Sphere. Each successive Captain-General since Kenyon Marik has invoked Resolution 288 upon taking office, which allows the Captain-General certain powers in time of war or crisis. The Fourth Succession War began with the wedding of Hanse Davion to Melissa Steiner. The Capellan Confederation lost a large portion of their territory, and the Free Rasalhague Republic was created, along with the St. Ives Compact. The Free Worlds League remained out of most of the conflict, losing several worlds near Terra to a Tikonov offensive towards the end of the war.

While the other Inner Sphere Houses were exhausted from the war, the Free Worlds League remained completely intact, ready to exploit the economic boom after the peace was signed. League Traders began to do business with all the major Houses, and the Free Worlds League soon had one of the strongest economies in history. In 3030, the Duchy of Andurien seceded from the League, and allied itself to the Magistracy of Canopus, precipitating the War of Andurien Secession. The war dragged on for years, and Captain-General Janos Marik was killed in a bomb blast in 3035 that also severely injured his son Thomas. Thomas was taken to Terra and rehabilitated by ComStar in secret, until his return in 3036. He assumed the post of Captain-General and League forces quickly forced Andurien into submission. Thomas has since been instituting reforms in the League to break some of the diplomatic gridlock that paralyzed it for so long, keeping it from being an effective and unified force.