Liftoff is scheduled for 6:45 a.m. EDT from Launch Complex 41, with a three-hour window. There is a 55% chance of rain Tuesday, according to Accuweather.com.
The launch will be streamed on ULA's website.
The top-secret payload is for the U.S. Space Force Systems Command.
On Saturday, ULA rolled the rocket to the launch pad from the Integration Facility.
The 196-foot-tell Atlas V has had only one other launch this year: Boeing's Starliner Crewed Flight test in June to the International Space Station. The two-person crew remains aboard the ISS as testing on the Starliner capsule continues.
Tuesday's launch will be ULA's 100th national security mission launch.
An additional 15 commercial and non-defense missions, including crewed launches, are planned for the Atlas V.
The Atlas 5 was first launched in 2002 with a commercial communications satellite.
"This is a bittersweet moment for us," Col. Jim Horne, senior materiel leader of the Launch Execution Delta, said. "I've had the privilege of sitting console for the very first national security Atlas launch in 2007, STP-1 [Space Test Program 1] in March of that year and here we are with our last national security Atlas.
"I do want to say thank you to the entire team, the government and ULA. It's been a strong partnership over these last almost 20 years on this program and this has been our workhorse vehicle."
The Atlas rocket's five solid rocket boosters are mounted around the core rocket.
After the USSF-51 launch, ULA will concentrate on its second certification flight of the Vulcan rocket after a successful debut in January. The Vulcan rocket will replace Atlas V and the now-retired Delta IV.
ULA is a joint venture between Lockheed Martin Space and Boeing Defense, Space & Security founded in 2006.
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