The Alberta Party, which set its sights on appealing to centrists on the political spectrum, failed to make a breakthrough in the provincial vote on Tuesday, despite the leader's popularity.
Notley's NDP held on to its traditional base in Edmonton, which it swept in 2015, but was losing in numerous surrounding municipalities, rural ridings and Calgary constituencies it captured four years ago.
"Today our great province has sent a message to Canada and the world - Alberta is open for business", Kenney said to cheers from around 1,000 supporters.
"I think any time we get an ally looking at pragmatic policy in this country ... that's a very positive step in the right direction for the country writ large", she said.
Unofficial numbers showed the Alberta Party eked out less than 10 per cent of the popular vote, compared with 54 per cent for the UCP and 33 per cent for the NDP.
Premier John Horgan offered his congratulations to Kenney via social media, "I look forward to working together in the interests of both of our provinces".
Jason Kenney, leader of the United Conservative Party, left, speaks as Andrew Scheer, leader of Canada's Conservative Party, gestures to the crowd during a campaign rally in Calgary, Alberta on Thursday, April 11, 2019. But Mr. Mandel, 73, was unable to win his own hoped-for seat in Edmonton-McClung, losing to New Democratic Party incumbent Lorne Dach.
He was elected Alberta Party leader in February 2018.
The campaign also served as a test with Alberta voters for Mr Kenney and his newly formed UCP, founded in 2017 with the merger of two right-leaning provincial political parties.
Polls have closed in the Alberta election and the governing New Democrats are hoping for second term over a United Conservative Opposition intent on seizing power.
Kenney, a 50-year-old former federal cabinet minister, vowed action on jobs and the economy.
This time around, the Progressive Conservatives are no more.
Nearly 700,000 people voted early in malls, airports, recreation centres, public buildings and an Ikea store.
The province, once a money-making dynamo thanks to sky-high oil prices, has been struggling for years with sluggish returns on royalties, reduced drilling activity and unemployment levels stubbornly above seven per cent in Calgary and Edmonton.
And depending on who the victor is, Prime Minister Justin Trudeau may have to deal with another conservative premier amid a federal election year.
Kenney has called the attacks a "fear-and-smear" red herring to distract from the NDP's economic track record of multibillion-dollar budget deficits and soaring debt.