He is the co-founder (with Dan Kurzius) and former CEO of Mailchimp, an email marketing company.

[6] He is the co-founder and former CEO of Rocket Science Group,[7] commonly known as Mailchimp[6] a CRM with 15 million users.

[11] In August 2022, Chestnut abruptly stepped down as CEO one month after sending a nearly 1,400-word email to a small group of employees identifying what he considered an alarming trend in his workforce: new hires introducing themselves in virtual meetings with their preferred pronouns.

[12] Chestnut has focused his company's resources in providing compensatory benefits for employees.

[13] Chestnut's motivation for his corporate policy stems from his being raised by his military father who had few economic opportunities.