He was born in Penticton, British Columbia, the son of James Thom and Elena Myrtle Fennel, he graduated from the Vancouver School of Art in 1947. He never went to architecture school but apprenticed at Thompson, Berwick & Pratt, where he quickly became recognized as an unusually gifted drawer and designer. He was the primary designer of the B.C. Electric Building (now known as The Electra) on Burrard Street, in Vancouver, British Columbia. and also designed notable houses in the Vancouver area, several of which won Massey Awards, the country's top award for architecture. In 1957, he became a registered architect at Thompson, Berwick and Pratt and a partner shortly afterward. He is one of the most admired Canadian designer of universities for his two masterpieces, Massey College and Trent University's riverside campus.
He established R.J. Thom & Associates in Toronto in 1963 and later the Thom Partership. He was a Fellow of the Royal Architectural Institute of Canada. In 1980, he was made an Officer of the Order of Canada. He had a lifelong battle with alcoholism and was eventually forced out of the partnership by some younger partners. He died at his office in 1986 after a bout of heavy drinking.