His families were followers of the prophet Te Whiti, but Ellison converted to Christianity before he graduated from high school. In school and college he was on provincial teams in rugby, (field) hockey and cricket. In 1919 he graduated from medical school and was appointed chief medical officer for the island of Niue, then in 1923 he was transferred to the Chatham Islands. Later, after post-graduate studies, he was posted to Samoa, Fiji and the Cook Islands, before being appointed director of the Division of Maori Hygiene in the New Zealand Department of Health in 1927. He was posted back to the Cook Islands in 1931 and stayed there for 14 years as chief medical officer and also commissioner of the high court, making tremendous progress in improving the islanders' health with very limited resources. In 1945 he retired from the civil service and went into private practice in Manaia, Taranaki province.
In addition to the OBE (for services to the Polynesian people), he received the King George V Silver Jubilee Medal and the Queen Elizabeth II Coronation Medal.
He married twice and had 10 children.