Using this mirror, Roman will capture stunning space vistas with a field of view 100 times greater than Hubble images, the US space agency said on Thursday.
"Achieving this milestone is very exciting," Scott Smith, Roman telescope manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.
Roman, set to launch in 2025, will peer through dust and across vast stretches of space and time to study the universe using infrared light, which human eyes cannot see.
The amount of detail these observations will reveal is directly related to the size of the telescope's mirror, since a larger surface gathers more light and measures finer features.
Roman's primary mirror is 2.4 metres across. While it's the same size as the Hubble Space Telescope's main mirror, it is less than one-fourth the weight.
The mirror weighs only 186 kgs thanks to major improvements in technology.
The primary mirror, in concert with other optics, will send light to Roman's two science instruments -- the Wide Field Instrument and Coronagraph Instrument.
The first is essentially a giant 300MP camera that provides the same sharp resolution as Hubble across nearly 100 times the field of view.
Using this instrument, scientists will be able to map the structure and distribution of invisible dark matter, study planetary systems around other stars, and explore how the universe evolved to its present state.
The coronagraph demonstrates technology that blocks out the glare of stars and allows astronomers to directly image planets in orbit around them.
If the coronagraph technology performs as anticipated, it will see planets that are almost a billion times fainter than their host star and enable detailed studies of giant planets around other suns, NASA said.
Roman will observe from a vantage point about 1.5 million km away from Earth in the direction opposite the Sun.