A career Foreign Service officer, Nealon held posts in Canada, Uruguay, Hungary, Spain, and Chile before assuming his post as Ambassador to Honduras in August 2014; Nealon also served as the deputy of John F. Kelly, while Kelly was in charge of the United States Southern Command.

After leaving his ambassadorship in 2017, Nealon was appointed assistant secretary for international engagement at the Department of Homeland Security by Kelly in July. During his time as assistant secretary, Nealon supported a policy of deploying Homeland Security agents abroad. He resigned his post on February 8, 2018, due to his disagreements with the immigration policy of Donald Trump,and, specifically, the withdrawal of temporary protected status for Hondurans.