A major critic of Charles I of England during the period of Personal Rule from 1629 to 1640, he was one of the Five Members whose attempted arrest sparked the First English Civil War in August 1642.

Haselrig approved the Execution of Charles I in January 1649, although he refused to participate in his trial, but later opposed Oliver Cromwell's creation of The Protectorate in 1653.

At the Battle of Cheriton, his men defeated Sir Henry Bard's cavalry charge, seriously weakening Ralph Hopton's army in the west.

On 30 December 1647 he was appointed governor of Newcastle upon Tyne, which he successfully defended, besides defeating the Royalists on 2 July 1648 and regaining Tynemouth.

[5] Between 1647 and 1650 Haselrig and his son brought a large amount of property in the north east which included the manors of Bishop Auckland, Middleham, Easingwoodborough and Wolsingham at a total cost of over £22,500.

He was one of the leading men in the Commonwealth, but he was antagonised by Cromwell's expulsion of the Rump Parliament, and he opposed the Protectorate refusing to pay taxes.

[5] Haselrig considered Cromwell to be a traitor to the cause after this as he was a staunch republican and opposed to all rule by a single person whether by hereditary succession or military might.

Portsmouth had strong naval traditions and had always maintained independence from the army; it benefited from defensible fortifications on its land side and the support of Admiral John Lawson ensured that the city would not fall easily to a protracted siege.

Whetham was a friend of General George Monck in Scotland who had the best forces in Britain at his disposal and who had declared himself for Parliament in October.

By the next day a declaration was posted calling for citizens to "restore Parliament to their former freedom, being the peoples indubitable and undoubted birthright".

The military government, which was now named the Committee of Safety, despatched a force hoping that pro-army members would open the gates.

[6] Despite Monck's guarantee of a pardon, Haselrig was targeted by the Royalist Silius Titus, who was also responsible for disinterring the bodies of Cromwell, Bradshaw and Ireton and having them ritually executed at Tyburn.

He was rash, "hare-brained," and devoid of tact and had little claim to the title of a statesman, but his energy in the field and in parliament was often of great value to the parliamentary cause.

He exposed himself to considerable obloquy by his exactions and appropriations of confiscated landed property, though the accusation brought against him by John Lilburne was examined by a parliamentary committee and adjudged to be false.

[5] In 1646, Haselrig purchased Auckland Castle, previously home of the Bishop of Durham, and replaced it with a new country house.

After the 1660 Restoration, the property was returned to the Church of England, and the new bishop, John Cosin, demolished Haselrig's home and constructed his own.