It opened in 1877 between Girvan and Challoch Junction, where it joined the Portpatrick Railway, which had already reached Stranraer from Castle Douglas.

Running through difficult and sparsely populated terrain, the G&PJR was never financially successful, and on two occasions it had to suspend operations to due unpaid debts to adjacent railway companies.

The little harbour at Portpatrick was the starting point for the short sea route from Scotland to the north of Ireland as early as 1620, connecting with Donaghadee in County Down, on the east side of Belfast Lough.

[1] While English destinations via Dumfries and Carlisle were dominant, Glasgow and Central Scotland were obviously significant, and when the Glasgow, Paisley, Kilmarnock and Ayr Railway (GPK&AR) reached Ayr in 1840 it was natural to consider whether an extension to Portpatrick, 60 miles (96 km) away, was feasible.

At Castle Douglas it made connection with other railways giving access to Dumfries and Carlisle, and therefore points in England, as well as Glasgow and Edinburgh by a roundabout route.

At this time money became scarce and subscriptions to the proposed railway were inadequate; nothing could be done for some years until in 1870 matters eased somewhat, and a contractor, Abraham Pilling of Bolton, undertook to build the line, accepting payment in shares.

Seemingly as an afterthought, the G&PJR applied for running powers between the proposed junction at Challoch and Stranraer; these were ratified by act of Parliament[which?]

Pilling too was struggling financially to continue his works due to a trade boom which made wages and materials prices escalate.

The G&SWR were approached and agreed to do so for 75% of gross receipts; this figure was thought excessive and talks were opened with Thomas Wheatley, who was successfully working the Wigtownshire Railway.

The CR was losing money on the Portpatrick Railway work and politely declined, assuming that the G&PJR would turn back to Wheatley.

[2] The G&PJR was responsible for rental payments to the Portpatrick Railway, calculated on the interest on the original capital cost of construction of their part of that line that they used.

The G&PJR was in desperate financial straits: at the shareholders' meeting on 30 April 1878 it was reported that the working arrangement with the G&SWR took over 90% of gross receipts.

Bouch had no difficulty in finding in favour of the Portpatrick Railway's case; the G&PJR lacking the cash to comply, held out.

Since 3 July 1879 the company's affairs had been in the hands of a judicial factor, James Haldane, appointed by the Court of Session.

On 31 January 1881 he gave notice to the G&SWR to terminate the working agreement, considered to be the greatest source of the G&PJR's financial problems.

The company's fortunes revived with this arrangement, and at a shareholders' meeting on 31 October 1884 a 1% dividend to Class A Debenture Stock holders was announced.

Feeling that the G&PJR was weaker, the G&SWR gave six months notice of termination of the working agreement, and the withdrawal of the £20,00 subscribed under the 1882 act.

He immediately ordered two 0-6-0 locomotives to be built new, and to cover while they were constructed, he acquired three second hand 4-4-0 engines and passenger and goods vehicles from the North London Railway (NLR).

I wonder who in Bow Works ever imagined that their prosaic suburban tank engine would one day run a boat express over a Scottish moor?

The G&PJR was authorised to borrow £10,000 for repair works on Pinmore Tunnel and the Stinchar Viaduct and for other expenses prior to the transfer.

In May 1887 two new 0-6-0 tender engines were delivered, emblazoned with the Ayrshire and Wigtownshire Railway (A&WR) lettering, having been obtained new from the Clyde Locomotive Company.

Nonetheless the financial performance of the little company was poor; internal traffic was very limited and steamship competition, and the attitude of the G&SWR, affected external business.

[5] In G&SWR days the Manson tablet exchange apparatus had been installed on the line, enabling faster transits through crossing stations.

The Hughes 2-6-0s had been brought in and found advantageous, but Smith records: "At the end of March 1939 there came momentous news—official sanction for a 60 ft [18 m]; 4-6-0s to work to Stranraer."

[8] Diesel multiple units were introduced on passenger trains from November 1959, but they reverted to locomotive haulage in the period 1984 to 1987.

The passenger service was irregular (with large gaps between trains) up until 2015 as it operated mainly in conjunction with the Belfast ferry sailings and is constrained to some degree by the single track nature of the line (there are only three passing loops on the line south of Girvan, at Barrhill, Glenwhilly and Dunragit).

A recently completed rail study [10] commissioned by the SPT, Passenger Focus and SWESTRANS has recommended that the timetable be recast where possible to encourage new passenger business (mainly by providing a more evenly spaced service) and also to seek to develop new freight flows such as timber from Barrhill, grain from Girvan and intermodal traffic between Northern Ireland and Scotland (and beyond).

A timetable recast by current franchise holders Abellio in December 2015 increased the service frequency on the line by 50% (from six trains each way up to nine on weekdays, running every two hours) but removed the direct link to Glasgow via Kilwinning; all services now operate to/from Kilmarnock on weekdays & Saturdays (a limited number are extended from there to or from Glasgow) and terminate/start from Ayr on Sundays.