Before 1615 [his father] William Sydenham of Wynford Eagle in Dorset and [his mother] Mary Jeffrey [aged 21] were married.
On or before 8th April 1615 the date he was baptised, William Sydenham was born to William Sydenham of Wynford Eagle in Dorset and Mary Jeffrey [aged 22].
In 1637 William Sydenham [aged 21] and Grace Trenchard were married.
On 17th June 1644 William Sydenham [aged 29] was appointed Governor of Weymouth by Robert Devereux 3rd Earl Essex [aged 53].
On 10th August 1644 William Sydenham [aged 29] and Anthony Ashley-Cooper 1st Earl Shaftesbury [aged 23] captured Wareham, Dorset [Map].
In November 1644 [his mother] Mary Jeffrey [aged 51] was murdered. In August 1644, Dorchester was being successfully held for Parliament against a Royalist attack, with three of the Sydenham brothers believed to have been involved in the defence. At some time during or shortly after this event a group of Royalist soldiers went to Wynford Eagle, and there Mary Sydenham was killed by a Major Williams.
On 10th February 1645 [his brother] Francis Sydenham [aged 28] died after having avenged the murder of his mother Mary Jeffrey three months before. On 9th November a large Royalist force of 300 cavalry led by Sir Lewis Dyve appeared outside of the towns’ wall. They did not attack, but contented themselves with hurling insults at the soldiers of the Poole garrison. It was then that Francis saw a Major Williams amongst the hecklers, the very man who had murdered his mother three months earlier at Wynford Eagle Manor. Incensed, Francis Sydenham and sixty of his men rode out of Poole and headed straight for the cavaliers, who turned and fled. Francis chased them the 24 miles to Dorchester and once there turned to his men and cried 'Give the dragoons no quarter and stick close to me, for I shall now avenge my mothers' innocent blood or die in this place'. He then spurred his horse on and charged headlong into the terrified Royalists, fighting his way with grim determination towards Major Williams, whom he shot dead, and whose body fell under his horse.
In November 1645 William Sydenham [aged 30] was elected MP Melcombe Regis.
On 14th August 1649 William Sydenham [aged 34] and Colonel Fleetwood were appointed Governor of the Isle of Wight.
Samuel Pepys' Diary. 17th January 1660. Thence I went to Westminster, and met Shaw and Washington, who told me how this day Sydenham [aged 44] was voted out of the House for sitting any more this Parliament, and that Salloway was voted out likewise and sent to the Tower, during the pleasure of the House. Home and wrote by the Post, and carried to Whitehall, and coming back turned in at Harper's, where Jack Price was, and I drank with him and he told me, among other, things, how much the Protector [aged 33] is altered, though he would seem to bear out his trouble very well, yet he is scarce able to talk sense with a man; and how he will say that "Who should a man trust, if he may not trust to a brother and an uncle;" and "how much those men have to answer before God Almighty, for their playing the knave with him as they did". He told me also, that there was; £100,000 offered, and would have been taken for his restitution, had not the Parliament come in as they did again; and that he do believe that the Protector will live to give a testimony of his valour and revenge yet before he dies, and that the Protector will say so himself sometimes. Thence I went home, it being late and my wife in bed.
Around August 1661 [his wife] Grace Trenchard died.
On 13th November 1661 William Sydenham [aged 46] died.