The 52-year-old "The Great Gatsby" director is making his television series debut. He will helm the first two episodes as well as the series finale alongside Emmy-nominee Ryan, reported Billboard.
Academy-winner Catherine Martin will serve as costume and production designer.
Smith, 16, will play Marcus "Dizzee" King, a psychedelically talented and enigmatic graffiti artist from the South Bronx.
"The Get Down" is a mythic saga of how New York at the brink of bankruptcy gave birth to hip-hop, punk and disco, told through the lives and music of the South Bronx kids who changed the city, and the world forever.
It is Luhrmann's first musical endeavour since "Moulin Rouge!" The Sony Pictures Television-produced entry will premiere on Netflix in 2016.