The dawning of the twentieth century brought new hardships and challenges. Though famine and disease were curbed, the political situation seemed threatening. Agitation for Home Rule finally succeeded in Parliament. To counteract the Home Rule Bill, a Solemn League and Covenant was signed in Belfast in 1913. Reverend Witherow of Markethill First Presbyterian was among the first to sign.
Others were looking at the changes in Europe. At the outbreak of World War in 1914, many from the district enlisted in the army. A number paid with their lives in service overseas. Their names are recalled in the various churches in the area. Among those wounded was Sir Archibald Acheson of Gosford Estate.
After the First World War, political divisions and the Government of Ireland Act in 1920 led to the establishment of a separate parliament for the six counties of Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone: effectively, the state of Northern Ireland had been born. As a borderland area Markethill, and its surrounding district, suffered in 'The Troubles' that accompanied the setting up of the new state. It would suffer for many years after.
The outbreak of the Second World War had a significant impact upon the town of Markethill. Nissen huts were constructed on the commons to house the Home Guard and Gosford Castle became a base for American soldiers and German Prisoner's of War. Mullabrack House, too, served as a billet for the officers. Men and women of this district gave their lives in the conflict, their names added to those already remembered on plaques and memorials. Many people find it surprising that, though the Second World War prompted more radical change within the town and district, the First World War had the greater impact in terms of human life. Within the local churches, people will often note a longer roll of honour for the First World War than for the Second World War.
During the war and its aftermath Markethill, like any other town, was subjected to rationing and its economic prosperity put in doubt. The town suffered when the railway line was finally removed in the 1950s and in 1967 when a bypass road was constructed. Yet it was a time of happiness as the cinema and dance halls in the area provided light entertainment for the people of this town and district. Gosford estate was finally purchased by the Ministry of Agriculture and subsequently opened as a Forest Park. Then the population was estimated as approximately 1200 inhabitants.