The office of Lord-Lieutenant is military in origin and can be said to date from the reign of Henry VIII when it's holder was made responsible for the maintenance of order, and for all military measures necessary locally for defence.  By 1569 provision was made for the appointment of deputies.  Although by the Regulation of the Forces Act 1871 the Militia was removed from the Lord-Lieutenant's direct control, it was not until 1921 that the Lord-Lieutenant finally lost the power to call on all able-bodied men of the county to fight in case of need.

The traditional links with the armed forces have been preserved in a modern form in the association of the Lord-Lieutenant with the Volunteer Reserve Forces. Lord-Lieutenants' connections with uniformed organisations have led to links with other uniformed organisations, such as the police, fire and ambulance services and many voluntary bodies, such as Red Cross, St John Ambulance, St Andrew, the cadet forces and other national and local youth organisations.  In recent years the circles within which the Lord-Lieutenants' leadership role is exercised have come to include a wide range of matters, civil and defence, professional and voluntary.  Lord-Lieutenants are effective in such work largely because of their links to the Crown and the essentially voluntary and apolitical nature of their role.

From earliest days the Lord-Lieutenant has also been closely associated with the magistracy.  Until the 19th century he appointed the Clerk of the Peace.

The Queen appoints Lord-Lieutenants on the recommendation of the Prime Minister; and the Queen approves the appointment of the Vice Lord-Lieutenants.

The fundamental principle concerning the office of Lord-Lieutenant is that he is Her Majesty's representative in his county and consequently it is his first and foremost duty to uphold the dignity of the Crown.  In this role he has wide discretion in how he carries out the tasks expected of him and those he chooses to undertake.

In general the Lord-Lieutenant will be following the example of the Queen and other members of the Royal family if he seeks to promote a good atmosphere and a spirit of co-operation by the encouragement he gives to voluntary service, and to benevolent organisations, and by the interest he takes in the business, industrial and social life of his county and the voluntary activity that goes on in it.

A woman who is appointed to the office of Lord-Lieutenant is correctly addressed as Lord-Lieutenant.