The New Democratic Party (NDP) ended the Progressive Conservatives' (PC) 44-year rule of the province.
Political observers were stunned by the result, with one commentator saying: "Pigs do fly".
Alberta's Premier Jim Prentice, a former member of Tory Prime Minister Stephen Harper's cabinet, said he was stepping down from political life.
He only became premier in September, and called the snap election in April to seek a mandate after bringing in a tough tax-raising budget.
"My contribution to public life is now at an end," Mr Prentice said as he conceded defeat, blaming the collapse in oil prices and the subsequent financial problems.
A federal court has decided that the National Security Agency's bulk, warrantless collection of millions of Americans' phone records is illegal.
The sweeping decision from the Second Circuit Court of Appeals on Thursday represents a major court victory for opponents of the NSA and comes just as Congress begins a fight over whether to renew the underlying law used to justify the program.
That program "exceeds the scope of what Congress has authorized," Judge Gerard Lynch wrote on behalf of the three-judge panel.