Seaham's industrial fortunes fluctuated wildly. 1921 brought a disastrous coal strike and closure of the Bottleworks with the loss of 500 jobs. The mid twenties brought more misery, closure of the Engine Works, cuts in wages and the calamitous General Strike. But Centenary year, 1928 brought the opening of the million pound Vane Tempest Pit and the huge Castlereagh extension at the Docks, both begun in 1923. At the end of 1929 Vane Tempest had been drawing coals for two years and dock exports were at an all time high of 2,300,000 tons. But a year later, as the depression deepened, to ensure production at the new pit, 1000 men were paid off at Dawdon and Seaham Colliery was closed for two years.
1935 saw the end of Seaham as a ship owning port when the "Maureen" last of the once great Londonderry fleet of twelve ships, sailed to the breakers yard. But that same year hopes were high as a large industrial plant for the production of oil and spirit from coal opened near the new pit. Unhappily within two years it proved to be a costly failure.
By the end of the 1930's all three pits were working and Seaham, with only 8% unemployment, was better off than most towns.
Social changes were all for the better. A statue to the memory of the Sixth Marquess, and memorial to the wartime dead added greatly to the dignity of North Terrace. Aged Miners homes were built on the seafront and at Dawdon, Welfare Parks were completed at New Seaham and Dawdon.
But the most spectacular social success story was Seaham Urban District Council's provision of council houses to replace the festering 19th Century slums of the central area. From 1926 onwards, when infant mortality was double the County average, the mushroom growth of the Carr House Estate (Deneside) provided hope, homes with water closets, electricity, gardens and for the first time, recreational space for thousands. Milestones in its growth included two blocks of shops, a recreation ground in 1931, the All Saints Mission Church in 1932, old people’s houses (the Lawns) in 1933, Deneside Park in 1935, new Infants and Junior Schools in 1935 and 1937. 6,700 people had been rehoused in ten years; health was much improved.
Cleared areas were replaced with lower density houses and after the absorption of New Seaham into the Urban District in 1937, work began on transferring people from the old colliery houses to a new estate at Parkside. Altogether, 2,424 council houses had been built when war came.
New schools were another source of pride. In 1930 Seaham Girls Secondary School was opened in the Upper Standard building, and two new schools, Camden Square Intermediate and Seaham Intermediate, allowed the old age schools to become Junior schools. Rock House Educational Settlement, opened in 1931, provided much needed local culture, and was described by J.B. Priestley in 1933 as "a tiny spot of light in darkness".
Many Seaham folk remember the thirties as hopeful, happy, romantic and sporting. The town's three recreation grounds provided football, cricket, tennis and bowls. The cinemas were popular. Social evenings and dances were enjoyed in church and chapel halls; brass bands were to be heard practising in Miners Halls and playing in the parks. The Annual Carnival at Dawdon with its jazz bands, fancy dress competitions and decorated floats was a highlight.
The Seaham engineering company, Crompton and Harrison contributed quite a lot to the war effort, making components and repairing ships coming into the Seaham Harbour Docks. When the men were called up to join the war, local women were employed as welders. One of the most dramatic secrets of the war was the construction and assembly of the famous 'Mulberry Harbour' used in the D.Day landings in 1944. Seaham had a hand in that work as sections of the Mulberry Harbour were made at the Crompton and Harrison engineering works.