[3] Day started his career at the financial firm White, Weld and Company in New York City.

[4] In 1969, Day founded and managed the portfolio of the Cypress Partners LP, an early example of a hedge fund focusing on recognizing undervalued or overvalued companies.

[8] Day was also the founder and chairman of Oakmont Corporation, a Los Angeles-based, family-owned investment company founded in 1980.

Day's partners in the Foley Timber and Land Company included Howard H. Leach, Ken Langone and Henry Kissinger.

The timberland was purchased in 1994 from Procter and Gamble, and when it was sold in 2015, it was considered to be the "largest undeveloped tract of private property east of the Mississippi River.

[10] Day was an active philanthropist throughout his life making major individual contributions and through the W. M. Keck Foundation which he chaired and served as CEO of.

Keck Foundation, Day donated US$500,000 to the Martha's Vineyard Public Charter School, including US$400,000 for the purpose of creating a new science center named in honor of his close friend Vernon Jordan, a Civil Rights activist, lawyer, Democratic advisor, and longtime Martha's Vineyard summer visitor.

In October 2006, Day hosted a fundraiser with then-President George W. Bush, which raised US$1 million for the Republican Party.

[19] In 2015, Day donated $1.1 million to Super PACs supporting Republican presidential candidates Jeb Bush and Carly Fiorina.

[20] In 2006, Day was awarded the Decoration of Officier de la Légion d'Honneur from the Government of France.