South West Norfolk has been held solidly by Conservatives since 1964 but for twenty years before then, it had been ultra-marginal: Labour first held it from 1929–31, and Sidney Dye won it for Labour in 1945 with a tiny majority of 53, holding it at the 1950 election by 260. He lost it to Denys Bullard in 1951 by 442 votes and won it back in 1955 with a majority of 193. Dye died at the end of 1958, and in the by-election Albert Hilton retained the seat for Labour by a margin of 1,354 votes. In the 1959 general election that soon followed, his majority was cut to 78.
On 28 May 2009, Fraser announced that he would be standing down at the 2010 general election citing family reasons. This was after his expenses claims were highlighted in the Daily Telegraph; according to the newspaper, Fraser claimed £1,800 in public money for buying 215 trees and marking out the boundary of his second home in the constituency.
1885-1918: The part of the Municipal Borough of Thetford in the county of Norfolk, and the Sessional Divisions of Clackclose, Grimshoe, South Greenhoe, and Wayland.
1918-1950: The Municipal Borough of Thetford, the Urban Districts of Downham Market, East Dereham, and Swaffham, the Rural Districts of Mitford and Launditch, and Swaffham, and parts of the Rural Districts of Downham, Marshland, and Thetford.
1983-2010: The District of Breckland wards of All Saints, Besthorpe, Buckenham, Conifer, East Guiltcross, Haggard De Toni, Harling, Haverscroft, Heathlands, Mid Forest, Nar Valley, Necton, Peddars Way, Queen's, Swaffham, Templar, Thetford Abbey, Thetford Barnham Cross, Thetford Guildhall, Thetford Saxon, Watton, Wayland, Weeting, West Guiltcross, and Wissey, and the Borough of King's Lynn and West Norfolk wards of Airfield, Denton, Denver, Downham Market, Emneth, Ten Mile, Upwell Outwell and Delph, Watlington, and Wissey.
2010-present: The District of Breckland wards of Conifer, East Guiltcross, Harling and Heathlands, Mid Forest, Nar Valley, Swaffham, Thetford Abbey, Thetford Castle, Thetford Guildhall, Thetford Saxon, Wayland, Weeting, and West Guiltcross, and the Borough of King's Lynn and West Norfolk wards of Airfield, Denton, Downham Old Town, East Downham, Emneth with Outwell, Hilgay with Denver, Mershe Lande, North Downham, St Lawrence, South Downham, Upwell and Delph, Walton, Watlington, Wiggenhall, and Wimbotsham with Fincham Wissey.
As a result of the Boundary Commission's report which came into effect for the 2010 general election, South West Norfolk gained wards from neighbouring North West Norfolk including Walpole, Tilney St Lawrence, and Wiggenhall villages. It lost to Mid Norfolk the wards of All Saints, Buckenham, Burgh and Haverscroft, Haggard De Toni, Necton, Queen's, Templar and Watton, which included the villages of Necton, Great Ellingham and Watton.
The constituency includes Downham Market, Swaffham, Thetford, Outwell, Upwell, and Feltwell.
In order to meet the strict requirements in respect of the size of constituency electorates, the Commission recommended that Norfolk be considered together with Cambridgeshire and Hertfordshire as a sub-region of the Eastern Region. Accordingly, it is proposed the two District of East Cambridgeshire wards which comprise the village of Littleport be transferred from the constituency of North East Cambridgeshire. To offset this, the Borough of King's Lynn and West Norfolk ward of Walton would be transferred to North West Norfolk and eastern, rural areas would be transferred to Mid Norfolk. As the constituency would no longer be exclusively in Norfolk, it is proposed that it be renamed Thetford and Downham Market.