After the Treaty of Ripon he hurried back to stand for Parliament. In October 1640 Slanning was elected for both Plympton Erle and Penryn to the Long Parliament (in a way which was to give rise to charges of bribery), and chose to sit for Penryn. His was among the 59 names of the members posted for voting against the Bill of Attainder of Strafford. Seven other Cornish MPs also voted against the Bill, including Godolphin, Trevanion, and Richard Arundell of Trerice, who later married Slanning's widow Gertrude Bagg. In June 1641 he returned to Cornwall to resume his governorship of Pendennis Castle, but was back in London that winter, and in January 1642 was called to attend the House of Commons for sending letters to Francis Bassett in Cornwall for the arrest of the "Five Members", should they try to embark from a Cornish port, a charge that Slanning denied. He was still in the House in February and supplied it with information concerning "four Scottish merchants lately arrived in Cornwall", but probably left for Pendennis in April when many MPs withdrew. He was certainly in Cornwall when, on 9 August, he was barred from the Commons and ordered to attend the House as a "delinquent". On 25 August 1642, the Royalist commander Hopton entered Cornwall after separating from the Marquis of Hertford following their failed attempts to secure Wiltshire, Dorset, and Somerset. Hopton first visited Sir Bevil Grenvile at Stowe, Kilkhampton, then after brushing aside Bullers's Militia, headed for Pendennis on 24 September to confer with Slanning. Hopton appeared voluntarily before the assizes at Truro and after successful defence of his actions, began recruiting. In November 1642, Slanning formed one of the five Cornish regiments foot which was known as "the Tinners". The other regiments were formed under Colonel William Godolphin, Sir Bevil Grenville, Colonel John Trevanion and Warwick Mohun, 2nd Baron Mohun of Okehampton. Slanning and the first three of these were known as the "Wheels on Charles's Wain". A seventeenth century ode included the distich:  Slanning was released from his governorship of Pendennis Castle in 1643 and was succeeded by Sir John Arundell. Hopton first used his forces to make an unsuccessful attempt on Exeter then fell back on Plympton, took it, and invested Plymouth on 1 December.