Green’s Dictionary of Slang

to earn (a great deal of) money, often as mint it, to prosper.

1953		Aberdeen Eve. Exp. 21 Feb. 5/2: ‘Meet Mr Callaghan,’ a simple, little melody, has been minting money for song-writer Eric Spear.
1969		H.C. Rae Interview 29: I mean, at my age some girls are minting it as models, or courtesans.
1987		Economist 302 55/2: British banks are minting it (after a bad tax knock in 1984), though profits are not back to the levels of several years ago.
1989		QED Info. Services Critical Issues in Information Processing 229: When you make ROM of $8 or more for every dollar spent on management after everybody else is paid, you are minting it. The ROl's on that are indecent.
1998		M. McCarthy Cantab. Intl Dict. Idioms 256/1: Ice cream sellers are minting it as the unseasonal heatwave continues.
2003		Drum (Johannesburg) n.d. 4/1: Our story on young rich blacks (page 24) was not easy to put together because while we have a list longer than your proverbial arm of blacks who are minting it [etc].
2010		K. Lace Moonlighting [ebook] ‘He had lots of dosh. I mean, if this organisation was as big as I was led to believe, they must have been minting it’.
2015		Thirteen O'Clock Press I am Woman - I am Man 126: It was the perfect business. I was minting it and life was great.