Koyomi Tominaga's career began at the elementary school when she started playing volleyball at the age of 9.

In 2006, she was again chosen as a member of the national team to participate in the Under-18 Asian Junior Volleyball Championship.

After graduating from senior high school, she joined the V.Premier League club Pioneer Red Wings in 2008.

At the end of the 2013–14 season, she moved to the Ageo Medics after Tohoku Pioneer announced the termination of the Red Wings.

She played for Lardini Filottrano of Italian Serie A1 in the 2018–19 season, on loan from Ageo Medics.