The locomotive became famous in 1925 when the GWR lent it to the London & North Eastern Railway as part of trials against the LNER's then new A1 Pacific Class, a famous example being LNER 4472 Flying Scotsman. Running from King's Cross to Grantham, and King's Cross to Doncaster, it made the ascent from King's Cross to Finsbury Park regularly in less than six minutes, a feat that the Pacifics were unable to match. Pendennis Castle was also shown to be more economical in both coal and water on the test runs, its superiority in burning unfamiliar Yorkshire coal being measured at 3.7lb per mile.
Before returning to the GWR, the locomotive attended the second Wembley Exhibition between May and October 1925, displayed next to Flying Scotsman, with a notice proclaiming it to be the most powerful passenger express locomotive in Britain.
Back at Old Oak Common, it continued to run the routes to South Wales and the West Country until after the GWR was nationalised post-World War II to become part of British Railways. In August 1950 it was allocated to Gloucester shed, in March 1959 to Bristol (Bath Road), and its last shed allocation was Bristol (Saint Philip's Marsh). It was withdrawn in May 1964.
In May 1964 it was booked to work the London Paddington to Taunton leg of Z48 on Saturday 9 May, where upon arrival at Westbury it was discovered that it had melted its firebars from the heat of its fire, blamed on the provision of unusually high-calorific-value, carefully screened coal which caused the fire to burn white-hot as the speed exceeded 90 mph. This special coal had been sourced in a well-meaning gesture by British Railways in the knowledge that the Castle would be deliberately pushed as close to 100 mph as possible on its last scheduled service. As there was no standby Castle at Westbury and the closest Castle was in Taunton GWR 6999 Capel Dewi Hall took its train onto Taunton where 7025 Sudeley Castle took over for the journey to Plymouth.
Acquired from BR in 1964 by Mike Higson, she was moved to Southall Railway Centre and appeared at one of the Great Western Society's first open days in 1965. After then being purchased by a partnership of the Honourable John Gretton and Sir Bill McAlpine, the locomotive was moved in 1967 to the former GWR depot at Didcot, taking up residence in the disused lifting shop. As the Great Western Society assembled and moved its collection there, the Castle made rare excursions at Didcot, and made her next public appearance in 1971.
In 1972 she moved to Market Overton, Rutland. After that project failed, and having been welded to the track by disillusioned project members, she moved to Steamtown, Carnforth where McAlpine held controlling-ownership. Being built to the larger GWR loading gauge, she was unable to run on the former London Midland and Scottish Railway tracks, and with tension growing between the owners, it was agreed to sell her.
In 1977, Pendennis Castle was sold to Hamersley Iron, one of the largest iron ore producers in Australia, which intended to run her on its 240 miles (390 km) ore-carrying Hamersley railway in the Pilbara region of Western Australia. The company-backed Pilbara Railways Historical Society wanted a steam locomotive, and chairman Russell Madigan had promised them one, possibly Flying Scotsman.
When purchased by Hamersley Iron, Pendennis Castle was in need of considerable boiler repairs, which delayed her departure for Australia. On 29 May 1977, she made her farewell run in the UK, at the head of the Great Western Envoy from Birmingham to Didcot and return. The following day, she headed to Avonmouth, the port for Bristol, where she was loaded aboard the cargo vessel Mishref, and departed for Sydney on 2 June 1977.
The Mishref arrived in Sydney on 13 July 1977, and the locomotive was unloaded in Darling Harbour. She was then towed to the Eveleigh Carriage Workshops, where she was stored pending delivery to her ultimate destination, Dampier, Western Australia.
On 26 March 1978, Pendennis Castle was towed 100 miles (160 km) to Newcastle, New South Wales, and, together with three new GE C36-7 diesel locomotives, was loaded aboard the Iron Baron for the voyage to the Pilbara. On 29 April 1978, the locomotive was unloaded at Dampier. One week later, Hamersley Iron's Manager, Operations, Ian Burston, handed her over to the Pilbara Railways Historical Society.
On 12 October 1978, Pendennis Castle and two former New South Wales Government Railways S type passenger cars were named Rio Fe ("River of Iron") by Lady Turner, wife of Sir Mark Turner, chairman of Rio Tinto - Zinc Corporation, one of the major investors in Hamersley Iron. A brass plaque bearing the name Rio Fe was fitted above the locomotive's smokebox door. Pendennis Castle's first passenger carrying journey in Western Australia was a return trip from Dampier to Dugite on 7 November 1978, with the locomotive hauling two passenger cars.
Modified in 1980, Pendennis Castle was used for various other excursion trains on the company's ore-carrying railways. After further modifications, she was moved 1,050 miles (1,690 km) by road to Perth, where she had a historic reunion with Flying Scotsman on 17 September 1989. During the 1990s, after various difficulties and with the expiration of the boiler certificate, she made her final run on 14 October 1994. Hamersley Iron being unprepared to pay a repair bill of 240,000 Australian Dollars, and repairs becoming uneconomical after fully electronic signalling was installed - which would have required the locomotive to run behind a GE Transportation Systems Dash 9 - resulted in the locomotive being stored for several years.
In 1999, Hamersley Iron's parent, Rio Tinto, decided to find a secure home for the locomotive, an idea conceived and executed by Adrian Lumley-Smith who at the time was on secondment from the UK to Hamersley Iron. An agreement was reached with the Great Western Society for the locomotive was donated. It was moved by road from Dampier to Perth, before being shipped to Bristol on 24 April 2000, and ultimately the Didcot Railway Centre.
Restoration was started in 2005, with the original intention that a return to main line service would take place in 2008. As of the start of 2018, restoration is still ongoing. It was announced in an issue of Steam Railway magazine that plans to run Pendennis Castle on an excursion trip with preserved sister 7029 Clun Castle to commemorate the end of steam and the end of the Castle Class on the Western Region in March 1967 would not go ahead as 4079 would not be restored in time for the anniversary in 2017. It was later announced that following Didcot's decision to withdraw from running on the mainline, on completion 4079 will only be run at Didcot and on heritage railway's.