He was too young to have been a member of the very first Hellfire Club founded by Philip, Duke of Wharton in 1719 and disbanded in 1721, but he and John Montagu, 4th Earl of Sandwich are alleged to have been members of a Hellfire Club that met at the George and Vulture Inn throughout the 1730s.

According to the 1779 book Nocturnal Revels, on the Grand Tour he had visited various religious seminaries, "founded, as it were, in direct contradiction to Nature and Reason; on his return to England, [he] thought that a burlesque Institution in the name of St Francis, would mark the absurdity of such sequestered Societies; and in lieu of the austerities and abstemiousness there practised, substitute convivial gaiety, unrestrained hilarity, and social felicity".

The first meeting of the group known facetiously as Brotherhood of St. Francis of Wycombe, Order of Knights of West Wycombe was held at Sir Francis' family home in West Wycombe on Walpurgis Night in 1752.

The initial meeting was something of a failure and the club subsequently moved their meetings to Medmenham Abbey (about 6 miles from West Wycombe) where they called themselves the Monks of Medmenham.

About 1755 Dashwood founded the famous "Hell-fire Club", or "monks of Medmenham Abbey".
 Medmenham Abbey, formerly belonging to the Cistercian order, was beautifully situated on the banks of the Thames near Marlow, Buckinghamshire.
It was rented, from Francis Duffield, by Dashwood, his half-brother Sir John Dashwood-King, his cousin Sir Thomas Stapleton, Paul Whitehead, John Wilkes, and others to the number of twelve, who frequently resorted thither during the summer.
They had it rebuilt by the architect Nicholas Revett in the style of the 18th century Gothic revival.
It is thought that Hogarth may have executed murals for this building; none, however, survive.

Over the grand entrance was placed, in stained glass, the famous inscription on Rabelais' abbey of Theleme, "Fay ce que voudras", the "monks" were called Franciscans, from Dashwood's Christian name, and they amused themselves with obscene parodies of Franciscan rites, and with orgies of drunkenness and debauchery which even John Almon, himself no prude, shrank from describing.

Dashwood, the most profane of that blasphemous crew, acted as a sort of high priest, and used a communion cup to pour out libations to heathen deities.
He had not even the excuse of comparative youth to palliate his conduct; he was approaching fifty, and thus ten years older than Thomas Potter whom Almon describes as the worst of the set and the corrupter of Wilkes; he was nearly twenty years older than Wilkes, and two years older than "the aged Paul" (Whitehead), who acted as secretary and steward of the order of ill-fame, and was branded by Charles Churchill as "a disgrace to manhood".
As a contrast to Medmenham Abbey, Dashwood erected a church on a neighbouring hill, which, as Churchill put it in "The Ghost", might "serve for show, if not for prayer", and Wilkes was equally caustic in his references to Dashwood's church "built on the top of a hill for the convenience and devotion of the town at the bottom of it".
He was too young to have been a member of the very first Hellfire Club founded by Philip, Duke of Wharton in 1719 and disbanded in 1721, but he and John Montagu, 4th Earl of Sandwich are alleged to have been members of a Hellfire Club that met at the George and Vulture Inn throughout the 1730s.
According to the 1779 book Nocturnal Revels, on the Grand Tour he had visited various religious seminaries, "founded, as it were, in direct contradiction to Nature and Reason; on his return to England, [he] thought that a burlesque Institution in the name of St Francis, would mark the absurdity of such sequestered Societies; and in lieu of the austerities and abstemiousness there practised, substitute convivial gaiety, unrestrained hilarity, and social felicity".
The first meeting of the group known facetiously as Brotherhood of St. Francis of Wycombe, Order of Knights of West Wycombe was held at Sir Francis' family home in West Wycombe on Walpurgis Night in 1752. 
The initial meeting was something of a failure and the club subsequently moved their meetings to Medmenham Abbey (about 6 miles from West Wycombe) where they called themselves the Monks of Medmenham.
About 1755 Dashwood founded the famous "Hell-fire Club", or "monks of Medmenham Abbey".  Medmenham Abbey, formerly belonging to the Cistercian order, was beautifully situated on the banks of the Thames near Marlow, Buckinghamshire. It was rented, from Francis Duffield, by Dashwood, his half-brother Sir John Dashwood-King, his cousin Sir Thomas Stapleton, Paul Whitehead, John Wilkes, and others to the number of twelve, who frequently resorted thither during the summer. They had it rebuilt by the architect Nicholas Revett in the style of the 18th century Gothic revival. It is thought that Hogarth may have executed murals for this building; none, however, survive.
Over the grand entrance was placed, in stained glass, the famous inscription on Rabelais' abbey of Theleme, "Fay ce que voudras", the "monks" were called Franciscans, from Dashwood's Christian name, and they amused themselves with obscene parodies of Franciscan rites, and with orgies of drunkenness and debauchery which even John Almon, himself no prude, shrank from describing.
Dashwood, the most profane of that blasphemous crew, acted as a sort of high priest, and used a communion cup to pour out libations to heathen deities. He had not even the excuse of comparative youth to palliate his conduct; he was approaching fifty, and thus ten years older than Thomas Potter whom Almon describes as the worst of the set and the corrupter of Wilkes; he was nearly twenty years older than Wilkes, and two years older than "the aged Paul" (Whitehead), who acted as secretary and steward of the order of ill-fame, and was branded by Charles Churchill as "a disgrace to manhood". As a contrast to Medmenham Abbey, Dashwood erected a church on a neighbouring hill, which, as Churchill put it in "The Ghost", might "serve for show, if not for prayer", and Wilkes was equally caustic in his references to Dashwood's church "built on the top of a hill for the convenience and devotion of the town at the bottom of it".
Francis Dashwood, 11th Baron le Despencer