The corps probably has the longest continuous history of any youth organisation in the country, but like most British institutions it has evolved haphazardly. A few of the landmarks in its long development are summarised here for interest.

1856 - Sailors returning from the Crimean war established 'Naval Lads Brigades' in Whitstable and other ports. By the turn of the century there were 'Brigs' and 'Brigantines' in several towns.

1910 - The Navy League, a pressure group formed in 1895 (see Unit history) with the aim of influencing maritime thinking in Parliament and reminding the country of its naval history and dependence on the sea, decided also to sponsor a small number of these independent units as the Navy League Boys' Naval Brigade. This slowly expanded with the addition of other formations such as Sea Scout groups.

1914 - The Navy League applied to the Admiralty for recognition of its 34 Brigades. An officer on the staff of the Admiral Commanding Reserves granted this in 1919, subject to an annual efficiency inspection, and the title 'Navy League Sea Cadets' was adopted. There was 5 other Sea Cadet Corps, all much smaller.

1937 - Lord Nuffield gave £50,000 to fund the expansion of the Corps.

1939 - At the start of the war there were nearly 100 units with some 10,000 Cadets.

1942 - The Navy League's 1941 scheme for training Sea Cadets in T.S. Bounty, for service in the wartime navy, caught the Admiralty's imagination. The Admiral Commanding Reserves took over the training role in January 1942, HM King George VI became Admiral of the Corps, Officers were granted appointments in the RNVR and the Corps was renamed The Sea Cadet Corps. A huge expansion to 400 units and 50,000 cadets coincided in many towns with warship weeks so those newly formed units took the names of adopted warships. The Admiralty now paid for uniforms; equipment, travel and training while the Navy League funded sports and unit headquarters. Thousands of Bounty boys progressed into the Navy as Communications ratings, many returning to their units after the war ended. In the same year the Girls' Naval Training Corps was formed as part of the National Association of Girls' Corps, with units mainly in Southern England.

1943 - All units were given unit numbers in alphabetical order from "1" Aberdare to "381" York. Thereafter, units were numbered in sequence as they were affiliated to the Navy League, reaching 430 by the end of the war.

1947 - The Admiralty offered to take over the Sea Cadet Corps entirely. The Navy League disagreed but suggested continuation of its co-sponsorship of the Sea Cadet Corps as during the war. The conditions were now embodied in an agreement with the Navy League known as the Sea Cadet Charter. Amongst other items the Admiralty undertook to support a maximum of 22,000 cadets, to supply uniforms, boats, training facilities, travel expenses and limited pay to adult staff who retained their appointments in the RNVR (and in the later reorganisation of the RNR).

The Sea Cadet Council was set up to govern the Corps with membership from the Navy League and the Royal Navy, and a retired Captain took on the task of supervision first as secretary to the council and later as Captain Sea Cadet Corps. From the same date the GNTC expanded throughout the country. By the late 1950's there were more than 50 units and the name was changed to the 'Girls Nautical Training Corps'.

1955 - The Commandant General Royal Marines asked permission to form a Marine Cadet section which could be fitted into the existing organisation and the Council agreed to this. By 1964 the section had expanded from the original 5 detachments to 40. By 1955, 90 units had Marine Cadet Detachments.

1966 - It was proposed to amalgamate the three Girls' Corps into one national body, to be called the Girls' Venture Corps. The Girls' Nautical Training Corps, not wishing to lose its naval identity, asked the Navy League to take over its sponsorship and in 1963 it was affiliated to the Sea Cadet Corps, in many cases sharing premises with local units.

1976 - The Navy League was renamed the Sea Cadet Association since support of the Sea Cadets and the Girls' Nautical Training Corps had now become its sole purpose. At the end of the year the title of Admiral Commanding Reserves had lapsed and its functions, including responsibility for the Sea Cadet Corps were transferred to the Commander-in-Chief Naval Home Command (CINCNAVHOME) in Portsmouth. The Charter was revised and replaced by a Memorandum of Agreement.

1980 - On 31st March, the Ministry of Defence (Navy) approved the admission of girls into the Sea Cadet Corps within the overall ceiling of numbers. The Girls' Nautical Training Corps ceased to exist as a separate body and its units admitted to the Sea Cadet Corps to form Girls' Nautical Training Contingents in a number of units. This number, originally set at 120, was raised to 150 in 1983 and then, in 1986,all limits on Contingent numbers were removed by the Admiralty board and replaced by a limit of 35% of girls in the Corps overall. By late 1991 over 300 units contained girls.

1992 - The successful integration of the boy and girl cadets and their adult leaders over the previous eleven years led to the logical step of discontinuing the separate GNTC Contingents from 1st January. Sea Cadets, male and female, now became entitled to identical training. Adult Sea cadet staff, male and female, became entitled to the same opportunities, insignia, rank nomenclature and pay. In its Golden Jubilee year under this title, the Sea Cadet Corps numbered some 400 units once more with a rising total membership of around 16,000. Sea Cadet Headquarters also retained a supervisory role over 3 units in Bermuda and 1 in Malta.

1992 - At a conference in Portsmouth an International Sea Cadet Association was formed to encourage international exchanges, to foster the Sea Cadet ethos world-wide and to stimulate the formation of new Corps. Founder members were UK, BELGIUM, Bermuda, Canada, Germany, Holland, Japan, South Africa, Sweden and USA.

1995 - The Sea Cadet Association was reconstituted as a Company as well as a National Charity.

2004 - The Sea Cadet Corps and the Marine Society merged to become the biggest Maritime youth organisation