1885-1918: The Sessional Divisions of Horsham, Midhurst, and Petworth, and the civil parish of Crawley.
1945-1950: The Urban Districts of Horsham, Shoreham-by-Sea, and Southwick, and the Rural Districts of Chanctonbury and Horsham.
1950-1974: The Urban District of Horsham, and the Rural Districts of Horsham, Midhurst, and Petworth.
1983-1997: The District of Horsham.
1997-2010: The District of Horsham wards of Billingshurst, Broadbridge Heath, Cowfold, Denne, Forest, Holbrook, Itchingfield and Shipley, Nuthurst, Riverside, Roffey North, Rudgwick, Rusper, Slinfold, Southwater, Trafalgar, and Warnham, the District of Mid Sussex wards of Balcombe, Copthorne and Worth, Crawley Down, Slaugham, and Turners Hill, and the District of Chichester wards of Plaistow and Wisborough Green.
2010–present: The District of Horsham wards of Billingshurst and Shipley, Broadbridge Heath, Denne, Forest, Holbrook East, Holbrook West, Horsham Park, Itchingfield, Slinfold and Warnham, Nuthurst, Roffey North, Roffey South, Rudgwick, Rusper and Colgate, Southwater, and Trafalgar, and the District of Mid Sussex wards of Ardingly and Balcombe, Copthorne and Worth, and Crawley Down and Turners Hill.
The constituency (including its brief larger versions under other names) has been represented by members of the Conservative Party since 1880, making it the longest held Conservative seat and not normally with marginal majorities making it a safe seat, though just outside the top twenty Conservative seats sorted by majority.
Horsham has existed as a constituency for three distinct periods. It first sent members to Parliament in 1295. However, the constituency was abolished in 1918 to make way for Horsham and Worthing. In 1945 the constituency was recreated, until 1974 when Horsham and Crawley was created. In 1983 the constituency of Horsham was again created and has existed since.
Both candidates received the same number of votes, and both were declared elected, with petitions lodged against both. However, on 3 May 1869, Aldridge withdrew his claim to the seat allowing Hurst to be the sole MP.
Held due to the 1847 general election result being declared void on petition, due to treating, on 23 March 1848. After a further petition arising from the by-election, Vesey-Fitzgerald was declared unduly elected, due to bribery and treating by both him and his agents, and Fitzalan-Howard was declared elected on 4 September 1848.