Its streamlined, wind tunnel tested[1] design allowed it to haul long distance express passenger services at high speeds.

On 3 July 1938, Mallard broke the world speed record for steam locomotives at 126 mph (203 km/h), which still stands today.

After a period of testing, a solution to deflect the smoke was found and the modification was incorporated into Mallard's final design.

The speed was achieved during the downward grade of Stoke Bank, south of Grantham at milepost 90¼, between Little Bytham and Essendine stations.

Mallard was just four months old at the time of the record, and was operated by driver Joseph Duddington, a man renowned within the LNER for taking calculated risks, and fireman Thomas Bray.

[6] Upon arrival at London King's Cross, driver Duddington and inspector Sid Jenkins were quoted as saying that they thought a speed of 130 mph (209 km/h) would have been possible if the train did not need to slow for a set of junctions at Essendine.

This had been foreseen by the publicity department, who had many pictures taken for the press, in case Mallard did not make it back to Kings Cross.

The (Edwardian period) Ivatt Atlantic that replaced Mallard at Peterborough was only just in sight when the head of publicity started handing out the pictures.

However, the dynamometer car tracks the current speed every half second on a paper roll moving 24 in (610 mm) for every mile travelled.

Immediately after the run staff in the dynamometer car calculated the speed over five second intervals, finding a maximum of 125 mph (201 km/h).

[9]: 126  In 1948, plaques proposed and designed by Harry Underwood, a headmaster and keen steam enthusiast, were fixed onto the locomotive which stated 126 mph (203 km/h), and this became the generally accepted speed.

[14] Mallard's record has never been officially exceeded by a steam locomotive, although a German DRG Class 05 reached 124 mph (200 km/h) in 1936 on a horizontal stretch of track, unlike Stoke Bank, which is slightly downhill.

[18] New build project Pennsylvania Railroad 5550 which is constructing a brand new T1, has stated their desire to test the locomotive when completed to see if it can claim the speed record from Mallard.

[19] In 1948, shortly after the formation of British Railways, the decision was taken to test locomotives from all of the former 'Big Four' companies to find the best attributes of speed, power and efficiency with coal and water.

Mallard had emerged from Doncaster with a fresh coat of post-war garter blue livery, stainless steel numbers 22 with a small 'E' painted above them (for Eastern region), new boiler (its fourth) and third tender of its career.

Its final revenue earning service took place on 25 April 1963, after which the locomotive was sent to Doncaster Works for repair and restored to its original condition.

[5]: 283  Following a relaxation of the ban on steam locomotives in the UK in the 1970s, Mallard hauled a train from Stewarts Lane, Battersea to York via the Midland Main Line in on 12 April 1975 in preparation for the opening of the National Railway Museum, where it remained.

[5]: 285–286 [21] In July 2003, Mallard was put on display outside the National Railway Museum as part of the 150th anniversary of Doncaster Works.

In 2013, six preserved A4s, including two in the United States and Canada, reunited at the museum for the 75th anniversary of Mallard's recording breaking run.

In July 2019, it made its first outside appearance since 2014 when it was displayed at York station alongside a Class 800 Azuma as part of the latter's launch by the London North Eastern Railway.

[23] Hornby Dublo first produced a model of Mallard in BR loco green for their 3-rail OO gauge system in 1958.