The Gainsborough Constituency was created for the 1885 general election, after the extension of the franchise through the Representation of the People Act 1884 and the reorganisation of constituencies through the Redistribution of Seats Act 1885. From 1983, it was named Gainsborough & Horncastle Constituency. The modern Gainsborough Constituency was formed in 1997 when the constituency reverted to being 'Gainsborough' following boundary changes, which resulted in the loss of Horncastle.
The constituency is entirely within Lincolnshire and has a total population of 81,596. The main population centre is the town of Gainsborough, secondary to this is the village of Welton, to the north of Lincoln. The constituency also includes the market towns of Wragby, Market Rasen and Caistor and the various villages and rural communities of the West Lindsey District. The constituency also comprises a number of villages and rural communities of the Wragby ward of the East Lindsey District.
Sir Edward Leigh was first elected to represent the people of the Gainsborough constituency in the general election of 1983. At the 2017 general election, Sir Edward was re-elected with 31,790 votes (a majority of 17,023) out of 51,575 votes cast.