Horner, 54, announced Thursday he is leaving politics on Jan. 31 after a career that included terms as deputy premier and treasurer in the Progressive Conservative government.
He said it's time to call it a career after 14 years serving five premiers.
"The time has come for me to move on, and to let others take that (responsibility) on," Horner said outside his legislature office.
"There's going to be an election. There's going to be a byelection. There are things that need to happen and I think you need somebody who is going to be committed for that longer run."
Horner is in his fourth term representing the riding of Spruce Grove-St. Albert.
He was first appointed to cabinet under former premier Ralph Klein and, besides finance, worked in the advanced education and agriculture portfolios.
His family roots run deep in politics.
His grandfather, Ralph Horner, served for 31 years in the Senate for the Conservatives.
His father, Hugh Horner, was an MP before joining Peter Lougheed and the provincial PCs in 1967. He was with Lougheed when the PCs took over government in 1971 and launched the political dynasty that continues to this day. He worked closely with Lougheed as deputy premier.
Doug Horner's uncles, Jack and Norval Horner, along with cousin Albert Horner, were also elected to Parliament.
Horner worked in agriculture before he was first elected to the legislature in 2001.
As agriculture minister, he fought to reopen markets after the mad cow scare and, as advanced education minister, helped reduce duplication and promote co-operation among post-secondary schools.
Along the way, he forged a reputation as a consensus builder and gentleman politician.
He made one grab for the brass ring when he ran against Alison Redford and Gary Mar in the 2011 race to take over from Ed Stelmach as PC party leader and premier.
He finished third, but then became one of Redford's key lieutenants as well as her finance minister.
But the good times turned sour. Widening price differentials on oil prompted Redford to begin taking on billions of dollars in debt to pay for infrastructure.
The government changed reporting methods for the budget, which sowed confusion and ignited accusations that Horner was trying to hide bad financial news.
He was the point man for planned changes to public-sector pensions, which brought more accusations that the government was raiding worker nest eggs. The changes were eventually shelved.
Redford quit as premier last March amid an escalating scandal over opulent spending on herself and advisers.
Horner was later criticized when the auditor general reported that on Horner's watch, Redford and government MLAs used government aircraft to fly to personal and political events.
Horner acknowledged at the time that he was in charge of the planes, but said it wasn't his job to police his colleagues. He wrote an angry, defensive email to his caucus mates, which was subsequently leaked to the media.
He said Thursday he had been thinking of leaving for more than a year, but said he wasn't going because of problems from the Redford premiership.
"I won't deny it was a very difficult time," he said.
"(But) six months worth of negative out of 14 (years) in politics, I think one should expect that in any career."
He also made no apologies for running up $11 billion in debt to pay for infrastructure, something that will continue under current premier Jim Prentice.
"There is no business person that I have talked to, that is involved in a growing economy, that would tell you that you should do what we did in the '90s, which was to balance the budget by not building schools. It makes no sense."
Horner is not in Prentice's cabinet, but is a senior adviser for him on economic issues. The premier, speaking in Calgary, thanked Horner for his work, particularly as finance minister.
"Some of what we're dealing with in terms of the financial circumstances in this province right now were made a little easier by some of the very good work that Doug did."
Horner said he will return to the business world and will look at opportunities in Asia.
— With files from Bill Graveland in Calgary
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