On May 16, 2016, Steffen received his first callup to the United States national team for a camp based in Puerto Rico.Steffen did not make an appearance, but in January 2018, he was called again into camp for a friendly against Bosnia and Herzegovina, against which he made his debut. He made his second appearance in a friendly against Paraguay, recording his first clean sheet in a 1–0 victory, on March 27, 2018, at WakeMed Soccer Park in Cary, North Carolina. On June 9, 2018, at the Parc Olympique Lyonnais in Lyon, Steffen notched seven saves against eventual 2018 FIFA World Cup champions France, including back-to-back saves in the dying seconds of stoppage time against Nabil Fekir and Ousmane Dembélé, as the United States drew France 1–1.
Steffen's sister, Katy, played college softball at Penn State University's Brandywine campus. His other sister, Lexy, briefly played collegiate soccer at West Chester University, appearing in one game in 2015 as a goalkeeper. Zack Steffen has the phrase "forever my family", as well as four German words, tattooed on his rib cage in honor of his four younger siblings. He majored in marketing while at Maryland. He is the oldest of five children, with the others being brothers Ben and Cole.
Steffen is a co-founder of VOYCENOW Foundation, a non-profit organization that brings athletes and fans together to help the black communities in America and fight for equality.