MacKellar was born in Govan and worked as a draughtsman at John Brown's shipyard in Glasgow.
In 1913, he was a founding members of the Glasgow branch of the Association of Engineering and Shipbuilding Draughtsmen (AESD).
[2] He rapidly became one of the union's leading figures, in 1918 chairing its committee on the Whitley Reports.
[3] In 1938, MacKellar was appointed as the AESD's full-time organiser for the London area, then later moved to cover Scotland and Northern Ireland.
[3] In the 1953 New Year Honours, MacKellar was made an Officer of the Order of the British Empire.