Under the "Justices Act 1886", and further specific legislation, the establishment of a place for the holding of a court of petty sessions by the Governor in Council was deemed to establish certain other specific courts. The "Magistrates Courts Act 1921" confirmed the applicability of this legislation to magistrates courts. The Magistrates Court, Mount Isa was proclaimed by the Governor in Council on 30 September 1924.
Under the "Justices Act Amendment Act 1964", from 1 January 1965 onwards: all courts of petty sessions districts were deemed to be magistrates courts districts, all places previously appointed for the holding of courts of petty sessions were deemed to be places for the holding of magistrates courts, all clerks of petty sessions were deemed to be registrars of magistrates courts. Under this legislation, from 1 January 1965 onwards, the Court of Petty Sessions, Mount Isa was deemed to be the Magistrates Court, Mount Isa.
Under the "Magistrates Courts Act 1921", magistrates courts inherited the civil functions of small debts courts. With the abolition of district courts by this same Act, their jurisdiction increased to claims of 200 or less in the following civil actions: personal actions, actions for the unliquidated balance of a partnership account, actions for a distributive share under an intestacy or for a legacy under a will, equity suits for recovery of money or damages, actions for replevin concerning distress for rent, summary actions for ejectment between lessors and lessees, and complaints for maintenance. In 2000 Magistrates Courts hear claims to $50000.
Under the "Justices Act Amendment Act 1964", magistrates courts inherited the petty criminal jurisdiction of courts of petty sessions. Petty criminal jurisdiction is a complicated matter of law involving numerous Commonwealth and State statutes but can be briefly described as jurisdiction over summary trials of simple offences and committal proceedings for indictable offences sent to the Supreme Court.
Actions brought before a Magistrates Court are heard and determined by a magistrate sitting alone. From 2004 the Magistrates Court, Mount Isa is within the Mount Isa Magistrates Courts District.