County Mayo was a parliamentary constituency in Ireland, which returned two Members of Parliament (MPs) to the House of Commons of the Parliament of the United Kingdom from 1801 to 1885.
The constituency was created in 1801 under the Acts of Union 1800, succeeding the earlier County Mayo constituency in the pre-union Parliament of Ireland. Under the Redistribution of Seats Act 1885 it was divided into four new single-seat constituencies: East Mayo, North Mayo, South Mayo and West Mayo.
This constituency comprised the whole of County Mayo.
The elections in this constituency took place using the first past the post electoral system.
Browne was elevated to the peerage, becoming 1st Baron Oranmore and causing a by-election.
Brabazon's death caused a by-election.
Blake resigned by accepting the office of Steward of the Chiltern Hundreds, causing a by-election.
Browne's death caused a by-election.
On petition, Moore was unseated, causing a by-election.
Moore's death caused a by-election.
On petition, Browne and Tighe were unseated.
Parnell was also elected MP for Cork City and opted to sit there, causing a by-election.