Simnel was born around 1477. His real name is not known – contemporary records call him John, not Lambert, and even his surname is suspect. Different sources have different claims of his parentage, from a baker and tradesman to organ builder. Most definitely, he was of humble origin. At the age of about ten, he was taken as a pupil by an Oxford-trained priest named Richard Simon (or Richard Symonds / Richard Simons / William Symonds) who apparently decided to become a kingmaker. He tutored the boy in courtly manners and contemporaries described the boy as handsome. He was taught the necessary etiquette and was well educated by Simon. One contemporary described him as "a boy so learned, that, had he ruled, he would have as a learned man."
Simon noticed a striking resemblance between Lambert and the sons of Edward IV, so he initially intended to present Simnel as Richard, Duke of York, son of King Edward IV, the younger of the vanished Princes in the Tower. However, when he heard rumours that the Earl of Warwick had died during his imprisonment in the Tower of London, he changed his mind. The real Warwick was a boy of about the same age and had a claim to the throne as the son of the Duke of Clarence, King Edward IV's brother.
According to James A. Williamson, Simnel was merely a figurehead for a rebellion that was already being planned by the Yorkists:
He was merely a commonplace tool to be used for important ends, and the attempt to overthrow Henry VII would have taken place had Simnel never existed. The Yorkist leaders were determined on a serious push, rising of their party in England supported by as great a force as possible from overseas.
Simon spread a rumour that Warwick had actually escaped from the Tower and was under his guardianship. He gained some support from Yorkists. He took Simnel to Ireland where there was still support for the Yorkist cause, and presented him to the head of the Irish government, the Earl of Kildare. Kildare was willing to support the story and invade England to overthrow King Henry. Simnel was paraded through the streets, carried on the shoulders of "the tallest man of the time", an individual called D'Arcy of Platten.
On 24 May 1487, Simnel was crowned in Christ Church Cathedral in Dublin as "King Edward VI". He was about 10 years old. Lord Kildare collected an army of Irish soldiers under the command of his younger brother, Thomas FitzGerald of Laccagh.
The Earl of Lincoln, formerly the designated successor of the late King Richard III, joined the conspiracy against Henry VII. He fled to Burgundy, where Warwick's aunt Margaret of York, the Dowager Duchess of Burgundy, kept her court. Lincoln claimed that he had taken part in young Warwick's supposed escape. He also met Viscount Lovell, who had supported a failed Yorkist uprising in 1486. Margaret collected 2,000 Flemish mercenaries and shipped them to Ireland under the command of Martin Schwartz, a noted military leader of the time. They arrived in Ireland on 5 May. King Henry was informed of this and began to gather troops.
Simnel's army—mainly Flemish and Irish troops—landed on Piel Island in the Furness area of Lancashire on 5 June 1487 and were joined by some English supporters. However, most local nobles, with the exception of Sir Thomas Broughton, did not join them. They clashed with the King's army on 16 June at the Battle of Stoke Field and were defeated. Lincoln, Thomas FitzGerald and Sir Thomas Broughton were killed. Lovell went missing; there were rumours that he had escaped and hidden to avoid retribution. Simons avoided execution due to his priestly status, but was imprisoned for life. Kildare, who had remained in Ireland, was pardoned.
King Henry pardoned young Simnel (probably because he recognised that Simnel had merely been a puppet in the hands of adults) and put him to work in the royal kitchen as a spit-turner. When he grew older, he became a falconer. Almost no information about his later life is known. He died some time between 1525 and 1535. He seems to have married, as he is probably the father of Richard Simnel, a canon of St Osyth's Priory in Essex during the reign of Henry VIII.
In the 1972 BBC serial The Shadow of the Tower, Simnel was portrayed by Gary Warren.
In the 2017 Starz miniseries The White Princess, Simnel is portrayed by Max True.