[5] On Australia Day (26 January 2014), McKellar was appointed a Companion of the Order of Australia (AC) by Governor-General Quentin Bryce, for his service to science, particularly theoretical physics, as an academic, educator and researcher, through seminal contributions to scientific development organisations, and as an author and mentor.

In 1965, McKellar was invited by J. Robert Oppenheimer to become a member of the Institute for Advanced Study, Princeton, USA, which he undertook from 1966 to 1968.

At Princeton McKellar began his work on weak interactions, calculating the parity violation in expected in nuclei.

It was characteristic of this work that he was applying the current algebra techniques of particle physics to obtain results about nuclei.

[6] He was a founding member of the Australian Research Council, playing a key role in both its establishment and development.

[6] McKellar actively facilitates physics research, policy development and international collaboration globally, especially in the Asia-Pacific Region.