The Watford New Line was opened in stages by the London and North Western Railway from 15 June 1912 as part of a wider scheme of suburban capacity improvement and electrification.

The London and North Western Railway (LNWR) drew up a plan in 1907 to widen their line between Watford and Kilburn and continue on to Euston in tube tunnel.

The section of double track between Willesden Junction and Harrow & Wealdstone opened on 15 June 1912 and was used for local steam train traffic.

[a] The section of new line between Harrow & Wealdstone and Watford High Street opened on 10 February 1913, with a new station at Headstone Lane.

[8] Kensal Green station, between Queen's Park and Willesden Junction, opened on 1 October 1916 for Bakerloo line service.

[12] The final section of new line was constructed between Queen's Park and Chalk Farm, with two platforms provided at Euston for electric trains.

[19][20] The new system had solid state interlocking, but far fewer signals; as a consequence the maximum traffic capacity of the line was severely reduced.

[21][22] The original electrification was on a fourth rail system, similar to that now used by London Underground, which allowed LER trains to use the new line.

North of Harrow & Wealdstone, now the limit of LU operation, the fourth rail has in most places been dropped onto the sleepers and remains bonded, thus leaving the resistance of the current return path unaltered.

The fourth rail remains in the normal position from Queen's Park to Kilburn High Road up platform, where a trailing crossover between those two stations is maintained in use to allow reversal of Bakerloo line trains unable to gain access to London Underground at Queen's Park, due to planned work or other reasons.

The electricity grid Willesden substation in Acton Lane, Park Royal supplies 11 kV, three-phase power to ten substations on the line, located at Camden, South Hampstead, Queen's Park, Willesden, Harlesden, Wembley, Kenton, Harrow, Hatch End, Bushey and Watford.

[28] The rebrand cost £4,000,000 (equivalent to £14,804,150 in 2023) and was launched at Wembley Central station by Ed Stewart, Ian St John and Jimmy Greaves.

[28] In April 1994, in preparation for rail privatisation, the line became part of the North London Railways train operating unit.

In July 2023, TfL announced that it would be giving each of its Overground services, including that on the Watford to Euston route, new names by the end of 2024.

From Queen's Park the Bakerloo line branches onto dedicated tracks in tunnel via central London to Elephant and Castle.

This short branch line was closed in 1964 as part of the Beeching cuts; the empty trackbed is still visible at Harrow & Wealdstone adjacent to the eastern ticket office.