In England, Forster became the successor of Joseph Priestley as tutor in modern languages and natural history at Warrington Academy where he worked for two years.

Forster's pride and obstinacy caused him to fall out with many powerful men in England; after clearing some of his substantial debt with the aid of German patrons, he returned to Germany where he was professor at the University of Halle from 1780.

[13] By July 1751, Forster had arrived in Danzig (Gdańsk), where he was a candidate for ordination at the Reformed church of St Peter and Paul [de; pl].

[15] Forster was ordained in Königsberg in August 1753 and (possibly to please his father) accepted the post of parson of the Hochzeit-Nassenhuben parish just south of Danzig, starting there on 23 September 1753.

[22][23] In 1757, he wrote to the mathematician Leonhard Euler in an attempt to find a position at the new Imperial Moscow University; while he obtained a recommendation, nothing came of it because of the intensifying Seven Years' War.

[27] In 1765, Forster obtained leave from his pastoral offices and travelled to St Petersburg with ten-year old George, their expenses paid by Hans Wilhelm Rehbinder, a Russian in Danzig who recommended them to Count Grigory Orlov, the favourite of Catherine the Great.

[30] Forster also made scientific plans for the journey, which the Academy of Sciences in St Petersburg supported by providing him with reference books and instruments.

They reached the Kalmyk Steppe and Lake Elton (where Forster studied the salt industry), and inspected further settlements along the Volga and some of its tributaries before returning to St Petersburg in October.

[33] Forster lived made contact with other scientists, and his scientific observations from the journey were read in several sessions of the Academy of Sciences, introduced by the geologist Johann Gottlieb Lehmann.

After it was received by Orlov and Catherine, Forster was asked to draft regulations for the colonies that would include improved governance and some degree of autonomy, and finished this work in May 1766.

[45] Forster sold some items collected during the Russian expedition, including coins and fossils, and tried to find a salaried position in order to bring the rest of his family to England.

[49] In June 1767, Forster was appointed as tutor in modern languages and natural history at Warrington Academy, replacing Joseph Priestley, with an annual salary of £60.

[53][54] The geologist and science historian Victor Eyles described Forster's approach as "rather more scientific than some of his contemporaries" and giving a "sound grounding in the subject".

[60] Forster increasingly was in financial trouble, and started to teach French at the grammar school in Winwick, sharing the duties with his son, who taught the less able pupils.

[66][67] In the summer of 1770, Forster was invited to accompany the geographer Alexander Dalrymple on an East India Company expedition to Balambangan and planned to take George with him as a midshipman.

[70] The Catalogue of the Animals, described by ornithologist Elsa Guerdrum Allen as "the first attempt to cover American fauna", contains a section with "Directions for Collecting, Preserving and Transporting all Kinds of Natural History Curiosities", a guide for field studies.

The English parliament had not long before voted to spend £4,000 for a scientist to accompany Cook on the voyage, which was intended to go to the Scottish physician and naturalist James Lind, whom Banks had chosen as a member of his party.

[85] In a letter to the Prime Minister, Lord North, Sandwich asked him to support Forster, writing For these reasons I hope (if you have no objection of your own) that you will encourage Mr. Foster [sic] who from all hands is admitted to be one of the fittest persons in Europe for such an undertaking; he is ready to go at a moment's warning, is thoroughly satisfied of the safety of the ship and with her present accomodation, and having his son with him, who is a very able draughtsman and designer, will fully supply the chasm occasioned by Mr. Banks' having withdrawn himself from the voyage.

[93] In November 1772, the expedition spent three weeks in the Cape Colony in Southern Africa where Forster and the officers stayed with the trader Christoffel Brand.

[98][99] They stayed there for five weeks; while Forster complained it was not a good season for proper botany, he managed to describe at least 19 birds as well as several fishes and plants.

[102] After another tour to antarctic waters where the ship reached 71° 10′ southern latitude,[103] a Farthest South record that stood for almost 50 years,[104] they arrived at Easter Island in February 1774.

[108] It had been prepared on board and was rushed to publication while containing numerous errors, as Forster saw himself in competition with Banks and hoped to claim the discoveries of plant species for himself.

[114] However, Forster's sample chapters did not satisfy Sandwich, who claimed they were a narrative instead of a scientific work and asked Richard Owen Cambridge to correct them.

By late 1777, the book sales had not yet covered expenses and Forster was so deeply in debt that he was forced to sell some of his library to avoid debtor's prison.

[129] In February 1779, Forster was appointed professor of natural history and mineralogy at the University of Halle by Karl Abraham von Zedlitz, the minister of education of the Kingdom of Prussia.

[136][137] When Forster's supervision ended in 1788, he had added (in his own estimation) 2700 plants to the garden by using his international connections, for example those to Carl Peter Thunberg in Uppsala and William Aiton at Kew.

[142] He was more successful in terms of student attendance when he lectured in agriculture, and started publishing essays on cultivation, husbandry and general technology.

[145] Although Forster was by 1790 one of the most well-paid academics at Halle and had additional income from his books, he was often in financial difficulties as he spent his money amassing an extensive library with a special focus on maps and travel literature.

[160] In English-language scholarship concerned with the voyages of James Cook, the focus was for a long time on the elder Forster's difficult character;[d] his achievements and abilities were downplayed or ignored.

[167][168] The Australian art historian Bernard Smith called Forster a "pioneer of social anthropology" in 1985,[169] and Observations was described after the publication of a scholarly edition in 1996 as "the beginning of modern geography" by geographer David Stoddart and "one of the most important early examinations of the new Pacific archive" by anthropologist K. R.