The Lyran Commonwealth was formed in 2340 as an amalgamation of three smaller mutual-defense leagues: the Tamar Pact, the Federation of Skye, and the Protectorate of Donegal. The original Lyran capital was Arcturus. Shortly after the outbreak of hostilities between the Commonwealth and the Draconis Combine in 2407, however, the Lyran government moved to its current location on Tharkad. Soon after, leadership of the Commonwealth passed from Alistair Marsden to his only heir, Katherine Marsden Steiner, and remains in Steiner hands to this day.

The Lyran Commonwealth currently consists of about 400 worlds. During the course of the first three Succession Wars, it had been slowly but steadily losing ground to the Draconis Combine, which had taken almost a fourth of the original Lyran territories (including more than half the old Tamar Pact worlds along the Draconis Combine border). The Fourth Succession War saw the complete reversal of that trend, however, with the Lyrans capturing 53 Kurita worlds, while losing only 2.

One reason that House Steiner has remained a formidable force in Successor State politics even in the worst of times is that its domain includes some of the most richly endowed planets in the Inner Sphere. These include Alarion, Coventry, Donegal, Summer, Tamar, New Karlsruh, and Hesperus II. The latter is the site of the Star League's greatest surviving supply depots and the largest still-functioning 'Mech manufacturing and repair facility in the Successor States.