Party colleagues applaud Jack McConnell
Following a ballot at the party's Glasgow headquarters, he promised to put schools, health services, jobs, crime and transport top of his agenda.
Mr McConnell also pledged to stand by the 'partnership agreement' with the Liberal Democrats but hinted that some agreed policies may take some time to implement fully.
Saturday's result means that he can now go forward in the ballot for first minister in the Scottish Parliament on Thursday.
Although he was the only candidate, Mr McConnell had to secure more than 50% of the votes from his 54 Labour MSP colleagues and the 29 members of the party's Scottish Executive.
After securing 97.23% in the ballot, he formally took over from Henry McLeish, who resigned nine days ago amid an on-going row about his constituency office expenses.
In his acceptance speech Mr McConnell said he was "deeply honoured to receive such overwhelming support" from his colleagues.
He stressed that much work still needed to be done "to make devolution a success" and achieve "first class public services".
The new Scottish Labour leader said he lived in a "fantastic country" with strong economic, cultural, sporting and scientific traditions
Priorities first
But he cited "unacceptable" failures in government, particularly in relation to children, and vowed to make the country's public services fundamentally better.
Mr McConnell said that he would keep to "the people's priorities - schools, health services, jobs, crime and transport".
He also restated his commitment to proportional representation for local government and the use of private cash for public services although he failed to clarify any timetable.
"Let me be clear - these are processes and we must be about priorities," said Mr McConnell.
The first minister in waiting also spoke of his beliefs in the United Kingdom and "democratic open politics".
He said: "Anyone who knows me well, knows that these ideals and principles have driven me on all my adult life.
"If I am fortunate enough to be given the responsibilities of Scotland's first minister next Thursday, together we can use them to really make a difference."
Mr McConnell left the platform to rapturous applause but under no doubt that the hard work is only just beginning.
Ministerial team
On Tuesday he is likely to hold talks with the Liberal Democrats to ensure their support in the first minister ballot in the Scottish Parliament.
If this goes to plan, he will then finalise his ministerial team before returning to the chamber the following Tuesday to have the selections ratified.
But the process by which Mr McConnell was elected has been heavily criticised by opposition parties.
Speaking before Saturday's ballot, Scottish Conservative leader David McLetchie called the election a "sham".
"For a party that likes to deliver sanctimonious lectures about democracy and the new politics, Jack's non-election is a serious embarrassment which will be compounded by the fawning support of his Lib Dem lackeys in Thursday's election in the Parliament," said Mr McLetchie.
Scottish National Party leader John Swinney said Mr McConnell's unopposed candidacy was "a bleak day for democracy" and pledged to fight what he called Labour's "political hegemony" over the country.
Mr McConnell may also face a sweat over whether any further allegations about his private or political life will surface in the newspapers.
On Friday night the Labour Party warned newspapers to be responsible in their coverage of his leadership bid.
Party sources said that some editors were going to extraordinary lengths in to find a story about Mr McConnell's private life.