No woman has run a major mining company since Cynthia Carroll, who led Anglo American for more than five years until early 2013.
Gaines first joined the board of FMG in 2013 as its first female director, and became chief financial officer a year ago.
She today officially starts in the top job at FMG - its third CEO after founder Andrew Forrest and Nev Power - though her promotion was first announced on November 30.
As a result, former CEO Nev Power told MNN last week that the transition period had been very smooth.
"We've been working together for 12 months with her as CFO and she's been on the board for four years prior to that so we know each other reas...