Gillingham means a homestead of Gylla's family, from Old English ham (village, homestead) and ingas (family, followers), and was first recorded in 10th century as Gyllingeham.
The town is also referred to in old texts as Jillyingham Water, hence the pronunciation being Gillingham (the G sounds as a "J" as in the girls' name Jill).
The name Gillingham is recorded already in the Domesday book of 1086. It is said to have been named after a warlord, Gyllingas—from the old English gyllan, meaning "to shout". He was a notable man in Kent history as he led his warriors into battle screaming and shouting. At the time of the Norman Conquest, Gillingham was a small hamlet. It was given to his half-brother, Odo, Bishop of Bayeux, who rebuilt the parish church at Gillingham and constructed an Archbishop's Palace on land bordered by Grange Road, the ruins of which could still be seen in the last century. Gillingham itself, at the time, was a small hamlet, built around the parish church and surrounded by large farm-holdings, of which St. Mark's Parish formed part, being part of Brittain Farm.
William Adams mentioned Gillingham in his writings, saying: "... two English miles from Rochester and one mile from Chatham, where the King's ships do lie". Adams was baptised at Gillingham Parish Church on 24 September 1564.
The Strand was once owned by the Davenport family in 1635, the Davenport family included a Mayor of Gillingham, pie makers and key holders of Gillingham. The Davenport family had a road named after them in 1920. The Davenport Estate was in Ashford, Kent. The Estate comprised around 15000 acres and was called The Davenport Manor. The Davenport lost the Estate in 1889. The Davenport family was one of the Investors in the Chatham Dockyard.
In medieval times the part of Gillingham known as Grange was a limb of the Cinque Ports and the maritime importance of the area continued until the late 1940s. Indeed, a large part of Chatham Dockyard lay within Gillingham: the dockyard started in Gillingham and, until the day it was closed in 1984, two-thirds of the then modern-day dockyard lay within the boundaries of Gillingham. The dockyard was founded by Queen Elizabeth I on the site of the present gun wharf, the establishment being transferred to the present site about 1622. In 1667 a Dutch fleet sailed up the River Medway and, having landed at Queenborough on the Isle of Sheppey and laying siege to the fort at Sheerness, invaded Gillingham in what became known as the raid on the Medway. The Dutch eventually retreated, but the incident caused great humiliation to the Royal Navy.
The Seven Years' War began in 1756 and the government immediately gave orders for the defence of the dockyard; by 1758 the Chatham Lines of Defence were built. Over a mile long, they stretched across the neck of the dockyard peninsula, from Chatham Reach, south of the dockyard, across to Gillingham Reach on the opposite side. One of the redoubts on the Lines was at Amherst. The batteries faced away from the dockyard itself to forestall an attack from the landward side; the ships and shore-mounted guns on the river were considered sufficient to protect from that side. The lines of defence are now part of the Great Lines Heritage Park and also the Lower Lines Park (near MidKent College, Chatham Campus).
War with France began again in 1778, and once more it was necessary to strengthen the defences. Fort Amherst was the first to be improved; it was followed by work beginning in 1800 to add others at Fort Pitt, Chatham, plus Fort Delce and Fort Clarence (both in Rochester); later in the 19th century others were added, including one at Fort Darland in Gillingham. Within all these buildings a barracks was built to house the soldiers. All this work, and the expansion of the dockyard, meant that more homes were needed for the workers. The position of the Lines meant that this building could only happen beyond, and so New Brompton came into being. The population rose to 9,000 people by 1851.
Gillingham was still only a small village; eventually it, too, was swallowed up, and the name of the whole settlement changed to Gillingham. In the 1891 census its population was 27,809, and in 1901, it was 42,530.
In 1919, after World War I, a naval war memorial in the shape of a white stone obelisk was set up on the Great Lines, from where it can be seen for many miles. By 1901 Gillingham had a population in excess of 40,000. Additional structures were added in 1945 to commemorate the dead of World War II. Similar monuments stand in the dockyard towns of Portsmouth and Plymouth. In 1919 the Frederick Burton family, which owned the old brick works, sold it and migrated to Auckland New Zealand with their daughter Edith.
Gillingham has been the scene of two notable disasters: on 11 July 1929 a public demonstration by Gillingham Fire Brigade went wrong, resulting in 15 fatalities; and in the 1951 Gillingham bus disaster, 24 Royal Marine cadets aged 10 to 13 were killed in a road accident.