Regarded as one of the outstanding talents in America, the 23-year-old will join City in the summer of 2019 on a four-year deal. He will remain a Columbus Crew SC player for the early stages of their 2019 MLS Eastern Conference campaign, which begins in March, before officially becoming a City player in July.
Zack is the current USA No.1, winning six caps to date for his country. He began his career with Freiburg II, playing 14 times for the German side before moving to MLS side Columbus in 2016.
Steffen was voted the MLS Allstate Goalkeeper of the Year 2018 after keeping 10 clean sheets.