The Diocese covers the counties of Wigtonshire, Kirkcudbright and Dumfriesshire (the ancient Diocese of Galloway) and of Ayrshire, Lanarkshire, Renfrewshire, Dunbartonshire and West Stirlingshire south of the River Forth (the ancient Diocese of Glasgow).

The Diocese of Galloway, also known as ‘Candida Casa’ or Whithorn, meaning ‘White House’, was founded by St Ninian in the fifth century. The Diocese of Glasgow was founded in the sixth century by St Mungo, also known as St Kentigern, and patron saint of Glasgow.

St Mungo and St Ninian are respectively depicted on the left and right sides of the Diocesan shield.

On 9 January 1492, the Bishopric of Glasgow was raised in rank to be an Archbishopric. During the Scottish Reformation, the heritage and jurisdiction of the church passed into the hands of Church of Scotland. However, the small Scottish Episcopal Church continued the line of bishops of both dioceses, even though, in the 16th century, many of them held the office in title alone.

In 1697, the Diocese of Galloway was united with the Diocese of Edinburgh, and in 1708 the line of Archbishops of Glasgow fell vacant. At that point, episcopacy was abolished in the Church of Scotland and Episcopalians separated from the State church. The penal period in the 18th century, when the Episcopal Church was outlawed because of its adherence to the Stuart cause, was a low period for both Dioceses.

The line was revived with the appointment of Alexander Duncan in 1731 as Bishop, rather than Archbishop, of Glasgow. However, when Duncan died two years after his appointment as bishop, the see fell vacant once more. In 1787, William Abernethy Drummond became Bishop of Edinburgh and Galloway and Bishop of Brechin in a temporary personal union of the dioceses.

To this, he added the then vacant see of Glasgow in union with Edinburgh and Galloway. Within a year, Drummond gave way to John Strachan as the newly appointed Bishop of Brechin, and, in 1805, resigned from the united see of Edinburgh and Galloway (to Daniel Sandford) to focus on ministry in Glasgow. Drummond continued as Bishop of Glasgow until his death in 1809 when the see was reunited with Edinburgh and Galloway.

In 1837, James Walker, bishop of the triple see and Primus, gave way to Michael Russell to be the first modern Bishop of Glasgow and Galloway. In 1888, the counties of Selkirkshire, Peeblesshire and Roxburghshire, which were historically part of the Diocese of Galloway, were transferred from the Episcopalian Diocese of Glasgow and Galloway back to Edinburgh.