At present three to five-year-olds can get up to 1,140 hours of funded early learning and childcare a year.
But two-year-olds only qualify for funding if their parents meet a set criteria.
Mackay said: “Too many are being saddled with nursery fees that cost more than the mortgage.
“Too many cutting back on essentials and having their choices made for them, rather than having the independence that comes with a fairer system.
“That’s why, at this election, the Scottish Greens are proposing the biggest expansion of funded childcare for a generation.”
Mackay added the policy would help end the financial “cliff-edge” families currently faced when maternity leave ends.
