Robert Norman Houghton MacKellar was born in Govan, Glasgow, Scotland on 5 March 1890 and was articled to John Nisbet (1868-1951) in Glasgow from 1904 to 1911. During this period he also attended Glasgow School of Art. He then worked as an assistant to Thomas Martin Cappon (1863-1939) in 1911-12, to Charles James McNair (1881-1955) in 1913-14, and Miller & Black in Glasgow until c.1914.
In 1920 MacKellar moved to Newcastle-upon-Tyne where he was employed by James Thorburn Cackett (1860-1928) and Robert Robert Burns (1868-1954) at their practice Cackett & Burns Dick. In 1923 he was made a partner in the firm and by 1928 was the sole partner in the practice which by then had been renamed Cackett, Burns Dick & MacKellar.
MacKellar was a member of the Northern Architectural Association and in 1936 was elected a Fellow of the Royal Institute of British Architects (FRIBA). He died in Northumberland on 24 January 1973. At the time of his death, his address was given as 19 Edge Hill, Ponteland, Newcastle-upon-Tyne.