Stewart was born in Honolulu, Hawaii to Scottish parents, and grew up speaking four languages (English, Spanish, Portuguese and Hawaiian).
In 1952, he was drafted into the United States Army and served in Paris, France and Frankfurt, Germany as a translator.
After his army service, he enrolled at the University of California, Los Angeles, where he obtained his Bachelor's and Master's degrees.
[1][2] He was on the faculty of the Graduate Center of the City University of New York for over 25 years.
[1][2] In the late 1960s, he explored the sociolinguistics of multilingualism, introducing the notions of polycentric languages,[3] autonomy and heteronomy.