An opportunistic marine mammal survey through the Atlantic Ocean between the UK and Antarctica was undertaken in November and December 2020. The RRS James Clark Ross, was used to transfer cargo and personnel for the British Antarctic Survey (BAS) from the UK to Rothera Research Station, Antarctica for the start of the 2020/2021 summer season. The transit and station relief included stops at King Edward Point and Bird Island, South Georgia, Falkland Islands and Signy Research Station, South Orkney Islands. The journey took a total of 48 days. Two JNCC marine mammal observers were on-board as well as other dedicated observers. Marine mammal observations were made as opportunistic sightings and recorded along with geographical position and other metadata. This transit and subsequent survey was unique in its passage as few vessels journey down the centre of the North and South Atlantic and continue on down the Western Antarctic Peninsula. British Antarctic Survey will continue to make this journey twice a year and it could provide a platform for an annual marine mammal survey of the entire Atlantic Ocean.