Cassy O'Connor was on Tuesday declared victorious in the seat of Hobart - one of three Legislative Council divisions that went to the polls on Saturday.
It comes after the Greens boosted their lower house representation from two to five MPs at the March 23 state election.
The party picked up a 1.5 per cent swing, coinciding with the lower house increasing from 25 to 35 members.
Tasmania is widely acknowledged as being home to the world's first 'green' party - the United Tasmania Group which ran candidates at the 1972 state election.
Ms O'Connor resigned as Greens leader in July and left parliament for a tilt at the upper house.
She finished with 60 per cent of the vote after the provisional distribution of preferences, ahead of independent John Kelly.
There wasn't such good news for Labor, the party lost the seat of Elwick to local mayor and independent Bec Thomas.
In opposition since 2014, Labor now has 10 lower house members and just three in the 15-member upper house.
The Liberals held Prosser through another mayor Kerry Vincent, who replaces fellow Liberal Jane Howlett, to maintain the party's tally of four Legislative Council members.
The upper house has seven independents.
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