Matthew had been installed as Duke of Slovenia by his father in 1406, as part of the general decentralisation of the collective territories, and once he came of age and could govern without regents he largely pursued a policy of self-aggrandizement independently from Buda. This manifested in near-constant meddling in the technically separate Kingdom of Bosnia, as well as the pretense of overlordship over Croatia. Matthew would provoke two uprisings from Croatia, a distraction that John III could ill-afford as the chaos in Bohemia deepened. Elsewhere Matthew campaigned in Naples against James, a policy which was firmly halted by is elder brother SSigismund I's support of James against his Aragonese cousins.
Sigismund I died in 1437 without an heir and Matthew quickly picked up the support of his Luxembourg, Brandenburg and Bohemia-in-exile lords. His activities as Duke of Slovenia had not ingratiated him well with the Hungarian lords however and his election as king hinged on granting various rights to them, as well allowing them to dictate who would be installed in various government posts. He would then spend much of his reign railing against the appointments enduring a virtual civil war from 1438 to 1440. This concentration on Hungarian matters largely left Germany to organise itself against the Hussite menace. After a particularly devastating Hussite campaign into Wurzburg several German princes formed the Union of Cadolzburg which would pursue the war whilst Matthew liaised with the church authorities attempting to organise a lasting peace.
At the Battle of Jesenek in May 1438 the radical Taborites were finally defeated, removing the final obstacle to an agreement with the more moderate Hussites, the Utraquists. Matthew would re-enter Prague later that month, the first time a king of Bohemia had done so since 1419 and was properly crowned in Prague Cathedral in July. Peace with the Hussites, signed at Jihlava, secured their recognition of Matthew as their king but came with a price, and under the 'Prague Compacts' the Hussites were allowed to carry on with their worship and reform inside the Catholic church whilst denying the temporal power of the clergy. The Compacts would, despite occasional Papal denunciation, last until they were abolished by Emperor Charles VI in 1618 by which time Lutheranism had superseded Utraquism as Bohemia's dominant faith.
The rest of his reign was dedicated to restoring his 'majesty' in Hungary securing his income and restart a Neapolitan campaign which would deliver the Kingdom back into 'Bezier' hands. Again this largely meant leaving Germany to its own devices.
Matthew died in 1444 after seven years of rule. The collection of Luxembourg lands would all be inherited by his eldest son John II. John would however be unable to secure the Imperial title, largely thanks to his father's indifference to German affairs, and it would go to Frederick of Bavaria (Frederick III) instead.