[1] After graduation, Kierans worked for several years in the family fur and leather business before leaving to pursue a career in politics.

During his early life, Kierans was a passionate supporter of the cooperative movement and believed in the power of community organizing.

After serving as director of the school of commerce at McGill University and president of the Montreal Stock Exchange, Kierans entered provincial politics in 1963.

Initially a critic of Walter L. Gordon's economic nationalism, Kierans' experience in government changed his mind, and he became a believer in the need for state intervention in the economy.

In the 1980s, he became a familiar voice appearing with Dalton Camp and Stephen Lewis as part of a weekly political panel on Peter Gzowski's Canadian Broadcasting Corporation radio show, Morningside.