"[4] The son of a Basel Reformed priest, Seyler moved to London and then to Hamburg as a young adult, and established himself as a merchant banker in the 1750s.

He used his remaining funds to become the main shareholder, benefactor and effective leader of the idealistic Hamburg National Theatre, that became a leading cultural institution in Germany.

His theatre employed Lessing as the world's first dramaturg, culminating in the work Hamburg Dramaturgy that defined the field and gave it its name.

"[7] The company was to a large degree centered around his would-be wife and his close collaborator Konrad Ekhof, Germany's most famous actress and actor at the time, respectively.

He commissioned works such as Sturm und Drang by Klinger (which gave its name to the era), Ariadne auf Naxos by Benda and Alceste by Schweitzer, considered "the first serious German opera.

As a parish priest, his father did not have a fixed income but presumably lived off payments for church services, gifts and occasional government contributions.

In 1761 Seyler & Tillemann, acting as agents for their close business associate Heinrich Carl von Schimmelmann, leased the mint factory in Rethwisch from the impoverished Frederick Charles, Duke of Schleswig-Holstein-Sonderburg-Plön, a member of a cadet branch of the Danish royal family, to produce debased coins in the final years of the Seven Years' War.

[12] Seyler & Tillemann went bankrupt in the wake of the Amsterdam banking crisis of 1763 with 3–4 million Mark Banco in debts, an enormous sum.

Mary Lindemann notes, quoting from the 1765–1773 Imperial Cameral Tribunal proceedings and the memoirs of John Parish: Seyler was described as "a handsome bon vivant.

"[13] Despite suffering "a sensational bankruptcy for an enormous sum [...] neither of them [Seyler and Tillemann] had lost his good humour or his taste for light living.

"[18] Karl Mantzius noted: Nominally Johann Friedrich Löwen served as theatre director, but he had little influence, as Seyler as main shareholder and head of a three-member "administrative committee" (Verwaltungsausschuß) took all managerial decisions while Ekhof in practice assumed the artistic leadership.

"[7] While the National Theatre had avoided musical theatre, Seyler appointed Anton Schweitzer as music director, charged with adding opera to the spoken repertory, and the Seyler Company came to play a major role both in the development of a German opera tradition and in the promotion and popularisation of the Sturm und Drang dramas.

The lack of public interest in Hanover led to financial problems and when Ekhof in May 1770 also became seriously ill and unable to perform for some time, the situation worsened dramatically.

Wieland had just published his modern and ironic mirror-for-princes work, Der goldne Spiegel oder die Könige von Scheschian.

Adam Shoaff notes, In 1772 he reunited with his love interest Friederike Sophie Hensel, who had stayed at the Vienna Burgtheater for the past year, and they finally married in November 1772 in Oßmannstedt just outside Weimar.

After the palace fire in Weimar in May 1774, Anna Amalia was forced to dismiss the Seyler Company, and they left with a quarter year's wages and a letter of recommendation to Duke Ernest II in Gotha.

In Gotha Seyler met the Bohemian composer Georg Anton Benda and commissioned him to write several successful operas, including Ariadne auf Naxos, Medea and Pygmalion.

At its debut in 1775, Ariadne auf Naxos received enthusiastic reviews in Germany and afterwards, in the whole of Europe, with music critics calling attention to its originality, sweetness, and ingenious execution.

In 1776 Seyler also employed Goethe's close friend Friedrich Maximilian Klinger as a playwright and secretary, and he remained with the company for two years.

Over the next two years the Seyler Company was primarily based in Frankfurt and Mainz and travelled extensively to Cologne, Hanau, Mannheim, Heidelberg and Bonn.

On 17 June 1777 Seyler became director of the electoral Komödienhaus at Große Bleiche in Mainz; his ensemble then consisted of around 200 members, including his wife Sophie.

Reinhard Frost writes that the Seyler Company, with its literarily ambitious program, did not reach the broad audience in Frankfurt, which was accustomed to more popular entertainment.

[note 3] Some of the actors who worked under Seyler's direction at Mannheim were August Wilhelm Iffland, Johann David Beil and Heinrich Beck.

A report commissioned by Dalberg noted that Toscani belonged to "the weaker sex" and that Seyler was the director of a theatre company and should be held to a higher standard.

Earlier in that year she had published the romantic Singspiel or opera Huon and Amanda (or Oberon), inspired by a poem by their friend and collaborator Christoph Martin Wieland.

[34] Musicologist Thomas Bauman describes Oberon as "an important impulse for the creation of a generation of popular spectacles trading in magic and the exotic.

From 1798 he lived as a guest on the estate of the actor, his long-time friend and fellow prominent freemason Friedrich Ludwig Schröder in Rellingen in the Duchy of Holstein, where he died on 25 April 1800 at the age of 69.

John Warrack noted that: The Danish critic Knud Lyne Rahbek highlighted the fact that while he had never been an actor himself, he demonstrated a great enthusiasm for and refined knowledge of the arts.

[43] The actor August Wilhelm Iffland who worked under Seyler's direction said: His theatrical legacy eventually overshadowed the dubious reputation he had earned as a banker in his younger years.

The founding took place on 25 June 1774 in the Gasthof Zum Mohren, on the occasion of the Nativity of St John the Baptist, and Ekhof became the first Worshipful Master and Seyler the First Warden.