Martin Hamilton-Smith's shock announcement also prompted a front page headline of "Traitor" from the Adelaide Advertiser, but the former Liberal MP said on Tuesday that he was putting the people of South Australia first.
"What people deserve now is good, effective government," he said.
"They deserve certainty. My decision, I think, will do that."
Premier Jay Weatherill said Mr Hamilton-Smith would become a minister and take on the portfolios of investment and trade, defence industries and veterans affairs.
Mr Hamilton-Smith said Mr Weatherill had reached out to him after the state election earlier this year.
"This is a difficult decision. It is one I considered most carefully over number of days and weeks," Mr Hamilton-Smith said.
"It has been a tough decision, but it's right decision - and for that reason I'll sleep well."
Mr Weatherill says members of the government were unanimous in their support of Mr Hamilton-Smith joining the party and government.
[They are] supportive. They can see the obvious benefits...the obvious stability for the government on the floor of parliament," Mr Weatherill said.
Despite some voter backlash, Mr Hamilton-Smith's move was welcomed by a number of MPs on Twitter.
Opposition Leader Steven Marshall, however, was less supportive.
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