His time in Mannheim came to an abrupt end when he - seemingly fatally - wounded an opponent in a duel (allegedly because of jealousy), an act that forced him to leave Germany. It is hard to ascertain whether this is really what happened or just a Berlioz like cloak and dagger story. The only source for this tale is Beck’s pupil Henri Blanchard who published this account in a biography of his former teacher in the Revue et Gazette musicale de Paris (1845). According to Blanchard, Beck’s opponent had only feigned his death, but Beck was not to learn this for several years.

When Beck’s six symphonies op. 3 were published in Paris several years later (1762) the edition bore this title:

SEI SINFONIE / A PIÙ STROMENTI / Composte / Dal Sigr. FRANCESCO BECK / Virtuoso di Camera di Sua / A. S. L’ELECTOR PALATINO, / & Actualmente Primo Violino / d’ell Concerto di Marsilia. /OPERA TERZA. /...Chez Mr. Venier...’

From this it is sometimes inferred that the story of Beck fighting a duel was probably a hoax. If Beck, so the reasoning goes, had in all truth been forced to flee from the Mannheim court because he had killed someone in a duel, then he would not have possessed the audacity to openly advertise the fact that he used to be a chamber virtuoso with exactly the same court. However, seen from another angle this might corroborate the truth of Blanchard’s story. If Beck had indeed not killed the other man and he knew by then that everything had been a vicious prank and his opponent alive, then the story might be true after all.

Whatever the circumstances of his departure, from Mannheim Beck made his way to Venice where he appeared as a violinist and studied composition with Baldassare Galuppi. Galuppi, whose name as a composer has faded over the centuries, was between 1750 and 1765 the most performed opera composer in Europe. In Mannheim he was especially well known. Several of his operas (16 all together) were staged at Mannheim in the middle of the 18th century. Thus, Galuppi would have known Mannheim and the fact that Mannheim had the leading orchestra in Europe. This helps to explain why Beck after his dramatic flight had probably little difficulty to establish himself in Venice and become Galuppi’s pupil.

After three years in Venice he eloped to Naples with one Anna Oniga, his future wife and mother of his seven children. It seems that Beck's career as a composer started largely during his years in Italy; this may also explain why in many traits he shows himself to be quite his own man and rather independent of the typical Mannheim style.

After his sojourn in Italy Beck moved to Marseilles where he became concert master at the theatre orchestra. The exact date of his arrival in France is uncertain yet for the year 1757 the performance of one of his symphonies in Paris is documented. When Beck’s Parisian publisher Venier announced the publication of Beck's op. 3 in the November of 1762 he introduced Beck with these words: Actualmente Primo Violino del Concerto di Marsilia. Between 1757 and 1762 all of his 24 symphonies were published in rapid succession by Parisian firms. At least seven performances of Beck's symphonies are known to have been given in Marseilles in 1760 - 61.

In 1761 Beck moved from Marseilles to Bordeaux where he became director of concerts and was appointed conductor of the Grand Théâtre. He also was active as organist and teacher. Among his more prominent pupils were Pierre Gaveaux, Henri Blanchard and one Boscha. On 24 October 1774 he was appointed organist at St Seurin, Bordeaux, where his improvisatory powers won him admiration from the congregation.

By the time he was settled in Bordeaux Beck's best times as a composer were all but over. Much of what he wrote there seems to have been either lost or destroyed by Beck himself. His greatest success came in 1783 when he travelled to Paris to direct the first performance of his Stabat mater in Versailles.

As did many other composers, Beck wrote patriotic and revolutionary music during the Revolution, including a Hymne á l'Être Suprême. Nevertheless, in 1791 he got into trouble with the new authorities when through caustic remarks he openly ridiculed some overzealous partisans of the new spirit. He seems to have been a self assured, proud and stubborn man. When he was subsequently put before a revolutionary trial in his nightshirt, he answered questions concerning his attitude towards the revolution with the exclamation: "What could I do against the revolution!?"

Beck’s fortunes declined in the aftermath of the French Revolution. Whereas in the year 1791 he was still able to pay his mother in law a pension of 400 Livres, a few years later he was barely able to support his family.

In 1806 he sent the score of his Stabat Mater with a personal dedication to Napoleon – whether out of genuine admiration or in an attempt to ingratiate himself with the great Corse is hard to decide.

Beck had six daughters, two of whom died early. His only son was commander of a French corvette during the Napoleonic wars, and was held captive by the English for a long time.