Edward Sexby was born in Suffolk in 1616 but little else is known about his life before the English Civil War. Reportedly he was a son of a gentleman, had been an apprentice in London and may have had family connections to Cromwell. In 1643 he was a trooper in Cromwell's Roundhead cavalry regiment. He adopted Leveller ideals, was involved in the Putney Debates and resisted attempts to come into an agreement with Charles I. He was involved in capture of the king at Holdenby house in 1647.
Later that year he opposed the idea to disband the army and left but appeared as captain in 1649. He was appointed as governor of Portland. On 1650 he was sent to Scotland as a commander of infantry regiment and also raised a regiment to fight in Ireland. He took part of the siege of Tantallon Castle. In July 1651 he was accused of keeping money that should have gone to his subordinates. Same year he was sent to Bourdeaux to support Fronde rebellion with very few results and returned to England in August 1653.
Sexby opposed Cromwell's dismissal of the Rump Parliament in April 1653 and turned against him. He claimed that Cromwell had betrayed the Republic and had become an apostate and tyrant. He plotted with John Wildman and Richard Overton to overthrow the regime but had to leave 1655 when the conspiracy was discovered.
In the continent he began to negotiate with Spain in hope of raising an invading army to oust Cromwell's government. He even tried to convince exiled English Royalists that he was one of them, with mixed results.
In June 1656 Sexby visited England in disguise to judge the situation. Back in Flanders, he met another ex-soldier Miles Sindercombe who had also fell afoul of Cromwell's policies. Together they plotted the assassination in Cromwell in the hope that they could step into the following power vacuum and restore Republic in its former form.
Sindercombe returned to England, gathered a group of men to his aid and made various unsuccessful attempts of assassination. However, Cromwell's chief of intelligence John Thurloe found out about the scheme and the conspirators were captured on January 8 1657. Sindercombe was imprisoned in the Tower of London where he committed suicide.
When the plot became public, Sexby defended his views in pamphlet Killing Noe Murder, Briefly Discourst in Three Questions where he defended political assassination as a way to tyrannicide. He used a pseydonym William Allen. The pamphlet appeared in London in May 1657. Afterwards he secretly visited England couple of times to create a new conspiracy. He was captured on his visit on July 24 1657.
Cromwell interrogated Sexby and imprisoned him in the Tower. He became feverous, apparently went insane and died January 13 1658.