Built by the firm of Beyer, Peacock & Co along with No.4 Loch for the opening of the Port Erin line in 1874, No.5 Mona was named after the Island and became a steadfast performer for the Railway. Although very similar in design to and with the same nominal 3’ 0” diameter 120psi boilers as the three engines delivered the previous year for the opening of the Peel Line,  the side tanks and coal bunkers on engines No.4 and No.5 were six inches higher, allowing 385 gallons of water in each tank and 14 cwt of coal to be carried when full, essential for the more steeply graded and longer Port Erin line. The original livery was Holly Green with thick black lining edged in white. 

Larger rectangular shaped cab spectacles were fitted rather than the small round spectacles on the previous engines.  A tool box was formed as part of the side tank platework inside the cab, rather than being a separate wooden chest mounted on top of the left hand side tank forwards of the cab.  The injector overflow pipes were carried down the outside of the water tanks to finish below footplate level, bizarrely dissecting the still centrally mounted brass nameplates on No.4 Loch and No.5 Mona in such a manner that the letters of the names were spaced to allow for the pipe!  No.6 Peveril was built one year later to the same design as No.4 and No.5, albeit her name plates were positioned on the forward part of her side tanks. 

Modifications during No.5’s working life included the fitting of separate front rod-operated sanding gear in 1895 when a new 3’ 0" diameter boiler was also fitted.  New rear sandboxes were positioned inside the top outer corners of the coal bunker. With the water feed valves and the front sand boxes now occupying the two front boiler rings, there were no suitable places to re-position the original maker's plates which had curved backs and would not sit on a flat surface.  This is the reason why engines No.1-No.6 no longer carry original makers’ plates, the whereabouts or survival of the plates being unknown, presumed scrapped.  No.5 Mona was enlarged in 1914 by fitting a 3’ 3” diameter 160p.s.i. boiler and 480 gallon side tanks, increasing her tractive effort by one-third.  Similar changes had been made to engines No.4 Loch in 1909 and No.6 Peveril in 1911.  In the 1920s, the standard  livery remained Holly Green but with thinner black lining edged red. A mechanical lubricator driven off the left-hand leading crankpin was fitted in 1936, followed by steam and vacuum brake equipment.

Her present 3’ 3” diameter boiler with Ross “pop” safety valves, welded smokebox and cast chimney were supplied by Beyer, Peacock & Co., in 1946 when No.5 was painted in the post-war Indian Red livery with black lining edged yellow.  Carriage steam-heating supply equipment and the brass numeral on the left hand side tank were fitted at this time. She was withdrawn from service after closure of the Peel and Ramsey lines in 1968, carrying the Spring Green livery adopted by Lord Ailsa in 1967, with black lining edged white, and red painted connecting and coupling rods. Her last recorded mileage in 1964 was 1,896,370, but the locomotive was steamed as late as 1970 on departmental duties when a burst boiler tube saw her withdrawn from service and placed into storage.

The locomotive was purchased privately for preservation by the now-defunct Isle of Man Railway Society in 1978 but has remained unrestored and stored on the railway ever since, residing for twenty years in the old corrugated iron carriage shed at Douglas Station until it was demolished in 1999.  She was then relocated for a short while into the infill shed (a new covered area created behind the running shed).  Later the locomotive was to be enveloped in a sealed tent with sister No.9 Douglas at the back of the new carriage shed.  The locomotive returned to Government ownership in 2012 and in 2020 was removed from the sealed tent and brought by rail to Port St. Mary where expert contractors decontaminated the locomotive of asbestos, thereafter remaining in dismantled form.

In 2021 the Supporters’ Association announced their plan to cosmetically restore the locomotive for display purposes and work began on the frames in early 2022, the boiler and cabs being returned in December of that year.  Work progressed so the locomotive could take part in the 150th anniversary celebrations later that year, appearing in the Spring Green livery, that which she last carried in service.    Entering the railway museum following the festival, taking the place of No.6 Peveril in August 2023, it was announced in January 2024 that the work of the Association had been nominated for a prestigious Heritage Railway Association award.

Out on the south line in August 1968 at Castletown Station beside the advertisement hoarding, recently replicated by the station's Friends Of... group.