Marwah joined Scotiabank in Toronto as a financial analyst in 1978. Rising through the ranks, he went on to become the Chief Financial Officer (COO) of the bank in 1998. In 2002, he was made the bank's senior executive vice president and chief financial officer. In 2005, he was elevated as the vice chairman and chief administrative officer. In 2008, he was re-designated as vice chairman and chief operating officer of the bank—positions he held till his retirement in 2014.

He has served on the boards of many famous Canadian institutions such as the Toronto Star, the Toronto International Film Festival, the C.D. Howe Institute, the Royal Ontario Museum, the United Way Campaign, and the Hospital for Sick Children.

Marwah joins two other turbaned Sikhs - Defence Minister Harjeet Sajjan and Science Minister Navdeep Bains - in the Canadian parliament. While Marwah will be the only Indian-origin member of the Senate (upper house), there are over 20 MPs of Indian origin the Canadian House of Commons (lower house).