McMillan had a long playing career with more than 100 caps for the Bay as both a player and as captain and also played professionally in Japan.
"McMillan has built an extensive coaching CV at club and representative level in the Bay of Plenty and Wellington and has an impressive track record of success at all levels," Rogers said.
"His most recent success was leading the Wellington Under 19 team to win the title at the National Under 19 tournament."
McMillan said he was delighted to have been selected for his new roles.
"Bay of Plenty is a proud and passionate rugby community.
"I'm looking forward to leading a Steamers team that our community is proud of," he said.
The union also confirmed that Rodney Gibbs was reappointed as the Steamers assistant coach. The former Steamer has a long and distinguished coaching record in the Bay and for the past two years he has been the Steamers assistant coach.
"Clayton and I have coached together previously and achieved some very positive results. I am looking forward to working with Clayton again as we prepare for next year's campaign," Gibbs said.