The first known mention of Marcellus was in 1426, when he was arrested in Lübeck for selling forged letters of mortgage and subsequently escaped from prison. He then pretended to be a monk of the Church of the Holy Sepulchre until he met a fellow Franciscan friar. Then, with the help of his new friend, he pretended to be a Knight of St. John throughout northern Germany to collect the money for ransoming the King of Cyprus from Turkish captivity. He was again arrested but escaped. He was next found in the service of Henry Cardinal Beaufort, who was the envoy of Pope Martin V in Germany. When it came out that Marcellus was an escapee, he was arrested and imprisoned. He then escaped and fled, only to be captured and sentenced to be led through the streets and symbollically hanged. In August 1428, he was sentenced to imprisonment for life. He was kept in the tower of the Prince-Archbishop of Cologne's prison in Brühl, 12.5 miles (20 kilometers) south of Cologne. The conditions of his cell were so bad that he wrote and sent letters, which have been preserved, to the officers of the Prince-Archbishop to beg for his freedom. He was redeemed and freed after allegedly curing the Prince-Archbishop, , of a dangerous illness. By then, his deeds were so notorious that rumors of them were heard at the Council of Basel. In 1431, he was assigned to Neuss as a priest and canon but he was not happy there. In 1439 he fell into debauchery but he continued to minister until 1442. He seemed to have enjoyed great popularity in the parish. He then settled in Cologne and lived there for a while.