He continued his studies at St John's College, Oxford, and the British School at Athens (BSA), where he pursued a then-unusual interest in modern Greek historical anthropology.

He subsequently transferred as a staff officer to the Macedonian front under George Milne, where he undertook archaeological excavations at Chauchitza and helped to establish the rules and procedures for heritage protection in the area during wartime.

Following his demobilisation, he became the assistant director of the BSA from 1919 until 1922, took a fellowship in 1920 at New College, Oxford, and lectured widely in person and on BBC radio on archaeological matters.

During the inter-war period, he carried out excavations on behalf of the British Academy in the Hippodrome of Constantinople, and held temporary posts at the University of Bristol and at Bowdoin College in the United States.

He returned to military service shortly before the outbreak of the Second World War in September 1939, joining the Intelligence Corps as an officer and instructor.

Casson's academic interests and publications were eclectic: outside the archaeology of Classical Greece, he published the earliest major English work on Thrace, and wrote widely on Byzantine art.

[21] Casson's Macedonia, Thrace and Illyria, a work of historical geography, won the university's Conington Prize in 1924 and was published as a book in 1926.

[18] In 1927–1928, he excavated on behalf of the British Academy in the Hippodrome of Constantinople, where he demonstrated that the central line (spina) of the racetrack, consisting of obelisks including the Serpent Column, was built upon the ground rather than atop a masonry wall.

He was funded in his time at Bowdoin by Frank Gifford Tallman, and may have obtained his appointment through his connections to John Beazley, a noted British historian of classical art.

[29] In 1935, he published Steady Drummer, a memoir of his First World War experience called "brilliant and caustic" by Casson's obituarist John Myres.

[30] Before the outbreak of the Second World War in September 1939, Casson had joined the Army Officers' Emergency Reserve;[3] he was posted to the Intelligence Corps later in 1939.

27 Military Mission to arrive in Greece, reaching Athens shortly after the fall of the Albanian city of Korçë to the Greek counter-attack on 22 November.

[39] In 1943, Casson wrote Greece and Britain, a work which expounded upon the historical connections between the two countries in order to emphasise the importance of the wartime alliance between them.

Two other SOE agents, Stephen Maitland and Ivan Watkins Bert, as well as the MI9 attaché George Lionel Dawson-Damer, were also killed in the crash.

Casson was buried at Fairpark cemetery in Newquay: the details of his death were suppressed owing to the classified nature of his work.

[43] The Greek government organised a requiem mass in his memory at Saint Sophia Cathedral, London: Casson was the first British officer to receive this during the Second World War.

[42] Casson was elected a fellow of the Society of Antiquaries of London,[3] made an honorary member of the Bulgarian Archaeological Institute, and awarded the Greek Order of the Redeemer.

[7] In 1936, Casson's Oxford colleague Isaiah Berlin described him in a letter to the poet Stephen Spender as "an absolutely unimportant, unlearned, persecuted little buffoon who calls himself a communist and raises laughs at New College.

After Casson's death, the club's committee commissioned his wife, Elizabeth, to compile a bibliography of his writings in his memory, and hosted a reception in his honour on 29 November 1945 alongside the Anglo-Hellenic League at the Dorchester hotel in London.