Arnett was born in Toronto, Ontario, the son of Edith Alexandra "Alix" Palk and Emerson James "Jim" Arnett, who was a corporate lawyer and brewer, among other occupations. His parents were originally from Winnipeg, Manitoba, and he has roots on both sides of his family in Manitoba going back many generations.
Arnett has two older sisters and a younger brother. His father, a graduate of Harvard University, and a corporate lawyer served as the president and CEO of Molson Breweries from 1997 to 2000.
Arnett briefly attended Lakefield College School in Lakefield, Ontario, but was asked not to return after a semester for being a troublemaker. The Subway Academy II allowed him to take theatre classes at the Tarragon Theatre. He eventually graduated from Leaside High School and attended Concordia University, Montreal for a semester but dropped out.
As a teenager, he was encouraged by his mother to pursue an acting career. He auditioned for commercials in Toronto and enjoyed acting. In 1990, he moved to New York City to study acting at the Lee Strasberg Theatre and Film Institute. He appeared in plays in New York, and his first acting role was in Felicity Huffman's independent film Erie, which was filmed on the Erie Canal.
William Emerson "Will" Arnett served as an executive producer and voices the eponymous character BoJack Horseman of the Netflix series BoJack Horseman (2014–2020), for which he won an Annie Award at the 46th Annie Awards.
- Arnett and Alison Brie, both voice the most characters on the show.