Later working as a bank clerk in London, he embarked on archaeological research in an amateur capacity, visiting prehistoric barrows during his weekends and holidays to record their shape, dimensions, and location.

Moving to Devizes, in 1949 he entered the archaeological profession as an assistant to Christopher Hawkes and Stuart Piggott at the Victoria County History project.

From 1952 to 1972, Grinsell worked as Keeper of Anthropology and Archaeology at Bristol City Museum, during which time he continued his examination of barrows, focusing on those in south-west England.

His use of non-excavatory fieldwork influenced much British archaeology in the latter part of his 20th century, while his willingness to pay attention to other sources of information, such as folklore and place-names, has been deemed ahead of its time.

[4][5][6] Although he had developed his interest in prehistory through an examination of stone tools, he came to focus his attention on barrows,[2] recognising that these were among the least well understood prehistoric monuments in the British landscape and one of the few that could be studied from the position of an amateur.

[7] At the time, a number of antiquarians were still active in southern England, with Grinsell being encouraged in his interests by prominent figures in this milieu such as Eliot Curwen and Hadrian Allcroft.

[3] The archaeologist Nicholas Thomas later noted that despite Grinsell's status as an amateur, by 1932 he had established himself as being "without parallel" in British archaeology, with his accomplishments surpassing even the work of professionals like O. G. S.

[11] In 1932 Grinsell attended the First International Prehistoric Congress, held in London;[7] it was here that he was introduced to Crawford, with the two becoming close acquaintances, both sharing an interest in the use of maps as part of their archaeological fieldwork.

[13] Seeking an answer to this question, Grinsell conducted his only excavation, an examination of one of several bell barrows – collectively known as the known as the Devil's Humps – atop Bow Hill in Sussex.

[19] Thomas later stated that the Victoria County History of Wiltshire "represents the high point in [Grinsell's] combined powers of fieldwork, grasp of secondary sources and handling of a great deal of detailed information.

[2] In 1989, Grinsell published an autobiography; reviewing it for the Folklore journal, the historian Hilda Ellis Davidson praised it but noted that it does not "tell us very much about the inner life of the author".

[23] According to the archaeologist Paul Ashbee, Grinsell's "directness, whimsical humour and sound common sense were legendary, as were his walking, youth-hostelling and penchant for traditional afternoon teas of the cream variety.

[25] This recording was particularly valuable for archaeologists because it occurred before large numbers of British barrows were heavily damaged as a result of deep ploughing and land development.