The letter calls on Microsoft to ditch its $19.4 million (£14.7 million) contract with ICE in light of President Donald Trump's "zero-tolerance" immigration policy, in which families and children are separated after crossing the US-Mexico border illegally.
"We believe that Microsoft must take an ethical stand, and put children and families above profits", staff wrote, adding that the firm should immediately cancel its contracts with the "inhumane and cruel" agency.
The letter also called on Microsoft to draft a policy stating that the firm will not take on work for clients who "violate international human rights law". It added that Microsoft should be transparent about its work for the government.
The letter was posted before Nadella wrote to staff calling for Trump's "abhorrent" policy to end. But he downplayed the nature of the company's work with ICE, saying it was mostly just email, calendars, and other office software.
"This new policy implemented on the border is simply cruel and abusive, and we are standing for change", Nadella said, saying that as a parent and an immigrant "this issue touches me personally".
But he stressed: "I want to be clear: Microsoft is not working with the US government on any projects related to separating children from their families at the border."
Microsoft’s president, Brad Smith, also published a blog post titled, “The Country Needs to Get Immigration Right.”
Apple’s chief, Tim Cook told the Irish Times that the immigration policy was “heartbreaking”. Elon Musk, the chief executive of Tesla and SpaceX, tweeted that he was a “top donor” to the American Civil Liberties Union and said that “if there is some way for me to help these kids I will do so”. Sundar Pichai of Google, Dara Khosrowshahi of Uber and Chuck Robbins of Cisco also tweeted their opposition to the policy.