;Summary of results The Labour Party have held the seat since 1997, with the Conservative Party second-placed in each election. The 2015 result made the seat the 215th safest of the party's 232 seats, and thus the 18th most marginal seat. Thomas's majority has ranged from 2.3% in 1997 to 26.4% in 2017. Unlike Harrow East, it had always been won by the Conservative Party until Labour's landslide in 1997, when a swing of 17.5% was the eighth-highest swing in that election; it was the safest Tory seat lost to Labour. Its electorate produced another better than average result for Labour in 2001, with a swing from the Conservatives to Labour of 5.4%, bettered in only four seats out of 650. The 2005 challenge by future Conservative MPMike Freer produced a pro-Conservative swing of 4.5%. The 2010 inceptive seat saw a notional swing to the Tories in line with that nationally of 5.7%; in overall outcome the incumbent Labour MP's swing increased. The HarrowWest and East seats now present as less marginal than they were in previous decades. After a 1.1% swing to the Tories in 2015, the seat swung to Labour in 2017 by around 11%, with a local record number of votes for their incumbent candidate. At 34.4%, the Conservative vote share was the lowest in the seat's history, but the party received more votes in 2017 than in 2001 and 2010. ;Other parties UKIP, Liberal Democrat and Green Party candidates won less than 5% of the vote in 2015 and 2017, therefore forfeiting their deposits.
Boundaries
The constituency was created for the 1945 general election when the Harrow constituency was split into the new seats of Harrow East and Harrow West. It was reduced in size for the 1950 general election, when a third Harrow seat, Harrow Central, was created. The Boundary Commission review before the 1983 general election saw the London Borough of Harrow contained in two seats, resulting in Harrow West gaining parts of the abolished Harrow Central. The area covers the largely Georgian conservation area on a stand-alone, partly park-lined hill which hosts the famous Harrow School; it also includes the urban Harrow town centre, as well as suburban streets of homes with good-sized gardens and well-kept small parklands. The seat has fast London Underground links to the city centre and rail services, few dual carriageways and few high speed roads. Labour's union leaderTony McNulty lost the neighbouring Harrow East in 2010 to Blackman, who held on in 2015 and 2017. However, Harrow West's Gareth Thomas retained his seat with a reduced majority.
2010 boundaries
Reviewing such representation in North London, the Boundary Commission for England, as is custom agreed by Parliament, altered the area's limits to avoid malapportionment, as London's housing and rates of occupancy have altered. The western border district, town or neighbourhood of Pinner went to a new cross-Borough seat, Ruislip, Northwood and Pinner, making its source, based on a ward breakdown of the last result, and mirrored by local election results, a stronger seat for Labour; this was coupled with the inclusion of Marlborough ward which had returned many Labour councillors since World War II.
''Note: From 2010 historically Conservative-strong Pinner formed part of the new seat Ruislip, Northwood and Pinner. The 2010 changes are not based on the 2005 result as the constituency underwent major changes.
