Sindercombe was born in Kent and had been apprenticed to a surgeon. During the English Civil War he become a Roundhead and a Leveller. In 1649 he took part in the mutiny of his regiment and when it failed, he fled. In 1655 he re-appeared as a member of a cavalry regiment in Scotland and took part of the plot to take control of the local army. This failed as well and Sindercombe fled to Netherlands.
In Flanders he met another Leveller and anti-Cromwell plotter Edward Sexby in 1656. Sindercombe joined his plot to assassinate Cromwell in hope of restoring the Puritan republic as they saw it. Sexby supplied Sindercombe with money and weapons.
1656 Sindercombe returned to England and gathered a group of co-conspirators, including renegade soldier John Cecil, apparent conman William Boyes and John Toope, member of a Cromwell's Life-Guards. Toope gave plotters information about Cromwell's movements.
First Simdercombe rented a house in King Street in Westminster where they intended to shoot Cromwell when he rode past in his coach. However, they noticed that it would be a difficult place to escape after the attempt, so they abandoned the plan.
Next Sindercombe rented another house near the Westminster Abbey using a name "John Fish". Now he intended to shoot Cromwell with an arquebus on his way from Abbey to Parliament in the September 17 1656. However, when large crowd gathered outside, Boyes panicked and left and the attempt had to be abandoned.
Next Sindercombe's group intended to shoot Cromwell when he left for Hampton Court every Friday, as was his usual custom. They intended to shoot Cromwell's coach when it was going through a narrow passage. As it happened, Cromwell changed his mind in that particular Friday and plotters waited in vain.
Next idea was to shoot Cromwell when he was walking in the Hyde Park. They broke the hinges of the park gates to ensure easy escape and John Cecil begun to follow Cromwell and his entourage. However, Cromwell became interested of Cecil's horse and called him over. Cecil lost his nerve and could not shoot him. He afterwards claimed that the horse was ill so he could not have escaped.
After so many failed attempts, Cromwell's spymaster John Thurloe had noticed the would-be-assassins. He had already heard about the plot from his spies in the continent.
Sindercombe's next idea was to burn down Whitehall Palace and Lord Protector with it. Boyes made an explosive device out of gunpowder, tar and pitch and the group planted it in the palace chapel in January 8 1657. However, Toope, who had had a change of heart, had revealed the plan to authorities. When plotters left, guards disarmed the bomb.
Thurloe gave an order to arrest the plotters. Cecil was easily captured but Boyes escaped. Sindercombe fought the guards until one guard cut off part of his nose. Cecil and Sindercombe were sent to the Tower.
Cecil decided to tell all. With Toope's aid, Thurloe was able to reveal also Sexby's part in the plot and present his findings to the Parliament.
Sindercombe remained uncooperative. February 9 1657 he was found guilty of treason when both Cecil and Toope testified against him. Sindercombe did not want to face the humiliation of execution and committed suicide by poison in Tower in February 13 1657.