[3] After taking part in the Battle of Kilsyth on the covenanter side, Hamilton was sent by the Scottish Estates of the Realm to treat with Charles I at Newcastle in 1646, when he sought in vain to persuade the king to consent to the establishment of Presbyterianism in England.

On 26 September 1647 he signed, on behalf of the Scots, the treaty with Charles known as the "Engagement", at Carisbrooke Castle, and helped to organise the Second English Civil War.

[3] Hamilton retired to his estates on the Isle of Arran until the Scottish invasion of England during the 1650 to 1652 Anglo-Scottish War, when he acted as colonel of a regiment drawn mainly from his tenantry.

[3] At the Battle of Worcester on 3 September 1651, Hamilton was shot in the leg with a musket and he was taken back to his lodgings at The Commandery, Charles II's headquarters in that city, which had since been taken over by Roundhead soldiers.

[4] On his deathbed on 8 September, Hamilton had just enough energy to write the following farewell letter to his wife: "Dear Heart, YOV know I have been long labouring, though in great weakness, to be prepared against this expected Change, and I thank my God I find Comfort in it, in this my day of Tryal; for my Body is not more weakned by my Wounds, then I find my Spirit Comforted and Supported by the infinite Mercies and great Love of my Blessed Redeemer, who will be with me to the end and in the end.