Arriving in New South Wales in January 1788, he filled a number of roles in the newly established colony, including serving on picket duty, guarding convicts, and on the Criminal Court.

Clark's diaries, although never intended to be published, provide some of the most personal information about the early convict era in Australia, and are currently held by the State Library of New South Wales.

Having transferred to the 6th Company in 1783, he married Betsy Alicia Trevan, of Efford, Devon, on 23 June 1784, with a son, Ralph Stuart Clark, born on 23 August 1785.

[2] Arriving with the fleet in January 1788, Clark filled a number of roles in the colony, from guarding convicts to occasionally serving on the Criminal Court, which he heavily disliked.

[4] Following the death of Captain John Shea in February 1789, Clark was promoted to fill his role, and temporarily given the rank of first lieutenant by Major Robert Ross.

Owing to the lack of a natural harbour on the island, Sirius was forced to anchor off the coast, and land men and stores via smaller boats.

On Norfolk Island, Clark was made quartermaster general and keeper of the stores at Sydney Bay, and was later put in charge of the settlement at Charlotte Field.

Clark himself was killed in action some time in June 1794 off the coast of Hispaniola, while his son died of yellow fever towards the end of the same month.

[1][note 5] Spanning a period from 9 March 1787 to 17 June 1792, although with occasional gaps, Clark's diaries are some of the most personal writings still in existence from the early history of the colonisation of Australia.

[9] On arrival in Botany Bay, Clark was dismayed at the unsuitable conditions, which were far from what had been promised in England: "if we are obliged to settle here there will not a soul be alive in the course of a year".

[10] With the site for settlement subsequently moved to Sydney Cove, Clark remained distressed by the living conditions experienced by himself and his fellow officers, expressing in a letter to his family in England: "I never slept worse, my dear wife, than I did last night, what with the hard cold ground, spiders, ants and every vermin you can think of was crawling over me".

After one incident, in which a particularly troublesome female convict, Elizabeth Dudgeon, was punished for insulting a guard officer, he noted "she has long been fishing for it, which she has at last got to her heart's content".