LNWR office on The Quay,
, Ireland, 1910
The company was formed on 16 July 1846 by the amalgamation of the
Grand Junction Railway,
London and Birmingham Railway and the
Manchester and Birmingham Railway. This move was prompted in part by the
Great Western Railway's plans for a railway north from
Oxford to Birmingham.
 The company initially had a network of approximately 350 miles (560 km),
 connecting London with Birmingham, Crewe, Chester, Liverpool and Manchester.
The headquarters were at
Euston railway station. As traffic increased it was greatly expanded with the opening in 1849 of the Great Hall, designed by
Philip Charles Hardwick in
classical style. It was 126 ft (38 m) long, 61 ft (19 m) wide and 64 ft (20 m) high and cost £150,000
 (equivalent to £14,280,000 in 2016).
 The station stood on Drummond Street.
 Further expansion resulted in two additional platforms in the 1870s, and four more in the 1890s, bringing the total to 15.
The LNWR described itself as the Premier Line. This was justified as it included the pioneering Liverpool & Manchester Railway of 1830, and the original LNWR main line linking London, Birmingham and Lancashire had been the first big railway in Britain, opened throughout in 1838. As the largest
joint stock company in the United Kingdom, it collected a greater revenue than any other railway company of its era.
Grand Junction Railway acquisition of the
North Union Railway in 1846, the London and North Western Railway operated as far north as Preston.
 In 1859 the London and North Western Railway amalgamated with the
Lancaster and Carlisle Railway
Lancaster and Preston Junction Railway, amalgamated with
Lancaster and Carlisle Railway in 1859 and this combined enterprise was leased to the London and North Western Railway, the company now had a direct route from London to Carlisle.
In 1858 they merged with the
Chester and Holyhead Railway and became responsible for the lucrative Irish Mail trains via the
North Wales Main Line to
Holyhead and handled the Irish Mail.
On 1 February 1859 the company launched the limited mail service, which was only allowed to take three passenger coaches, one each for Glasgow, Edinburgh and Perth. The Postmaster General was always willing to allow a fourth coach provided the increased weight did not cause time to be lost in running. The train was timed to leave Euston at 20.30 and operated until the GPO instituted its own post train, wholly of Post Office vehicles, in 1885.
 On 1 October 1873 the first sleeping carriage ran between Euston and Glasgow, attached to the limited mail. It ran three nights a week in each direction. On 1 February 1874 a second carriage was provided and the service ran every night.
In 1860 the company pioneered the use of the water trough designed by
 It was introduced on a section of level track at Mochdre, between Llandudno Junction and Colwyn Bay.
The company inherited a number of manufacturing facilities from the companies with which it merged, but these were consolidated, and in 1862 locomotive construction and maintenance was done at the
Crewe Locomotive Works, carriage building was done at
Wolverton and wagon building was concentrated at
At the core of the LNWR system was the main line network connecting
London Euston with the major cities of
Manchester, and (through co-operation with the
Glasgow. This route is today known as the
West Coast Main Line. A ferry service also linked Holyhead to Greenore in County Louth, where the LNWR owned the 26-mile
Dundalk, Newry and Greenore Railway, which connected to other lines of the Irish mainline network at Dundalk and Newry.
The LNWR also had
a main line connecting Liverpool and Manchester with
Leeds, and secondary routes extending to
At its peak just before
World War I, it ran a route mileage of more than 1,500 miles, and employed 111,000 people. In 1913 the company achieved a total revenue of £17,219,060 (equivalent to £1,554,940,000 in 2016)
 with working expenses of £11,322,164
 (equivalent to £1,022,430,000 in 2016).
On 1 January 1922, one year before it amalgamated with other railways to create the
London, Midland and Scottish Railway, the LNWR amalgamated with the
Lancashire and Yorkshire Railway and at the same time absorbed the
North London Railway, the
Dearne Valley Railway and the
Shropshire Union Railways and Canal Company. With this, the LNWR achieved a route mileage (including joint lines, and lines leased or worked) of 2,707.88 miles (4,357.91 km).