Marshall was born Sayed Waqaar Hussain at Hutzel Women's Hospital in Detroit, Michigan. The son of Dr. S. Amjad Hussain, a cardiovascular-thoracic Surgeon and author and Dorothy Gladys (Brown), an Emergency Room RN. His paternal grandparents were Pakistani (then India) and his maternal grandparents were of German, English and Scottish descent. After Marshall was born, his family immediately relocated to his father's hometown of Peshawar, Pakistan in the Northwest Frontier Provence (now renamed Khyber Pakhtunkhwa). His mother then moved he and his sister (Natasha) back to Michigan during the 1971 Indo-Pakistani war. They resettled back in Peshawar after the war and the family permanently relocated back to the United States in 1974, first in Pennsylvania, where his brother (Monie) was born, and finally settling in Northwest Ohio in 1976. He attended grade school and high school at Maumee Valley Country Day School in Toledo, Ohio before moving on to Bowling Green State University and the University of Toledo. He then relocated to England after being awarded a place in the 3-year acting program at The Royal Academy of Dramatic Art in London (R.A.D.A.).
- Lentz, Harris M. (2001). Science Fiction, Horror & Fantasy Film and Television Credits: Television shows. McFarland. p. 1777. ISBN 978-0-7864-0952-5. Retrieved 9 June 2011.
- Jenkins, Jeffrey Eric (2009-08-15). The Best Plays Theater Yearbook 2007-2008. Limelight Editions. p. 404. ISBN 978-0-87910-366-8. Retrieved 9 June 2011.
- Morley, Sheridan (2 May 2001). "LONDON THEATER : 'Odd Couple':The Female Version". New York Times. Retrieved 9 June 2011.