John Reid has said he did not "lobby" for the job of home secretary and only agreed to do it after persuasion by Tony Blair.
Mr Reid has a steep learning curve ahead of him, he said
Mr Reid is thought to have wanted to become foreign secretary in Friday's Cabinet reshuffle.
The former defence secretary said he found the prospect of taking over his new department "daunting".
It is his ninth ministerial job in nine years, including health secretary, Commons leader and Labour chairman.
Speaking on the BBC's Politics Show Mr Reid, said: "When the prime minister asks me to do something I do it.
"I cannot say I was lobbying to get the home office. It is going to be a difficult task."
Home Secretary Charles Clarke was fired as home secretary after admitting he had lost track of more than 1,000 foreign criminals who should have been considered for deportation.
Sorting the freed foreign prisoners debacle out will be Mr Reid's top priority.
Writing in the News of the World, he said the current system "just isn't good enough".
He also admitted his new post was a "daunting" one and acknowledged that taxpayers should not expect Home Office systems to "cock up".
And he blamed the courts for often "thwarting" government wishes.
"My task is to reduce fear and increase fairness for all citizens," he added.
Mr Reid wrote that readers of the Sunday newspaper believed it was wrong for court judgements to put the rights of foreign prisoners ahead of the safety of UK citizens.
'Down to business'
"They believe that the government and their wishes are often thwarted by the courts. They want the deportation for foreign nationals to be considered early in their sentence, and are aware that this was overruled by the courts."
He said he also wanted to know why foreign prisoners had not been deported as soon into their sentence as possible.
Mr Reid said readers also "rightly believed" that systems in the Home Office "shouldn't cock up systematically - not just on an isolated case on issues as important as this".
The former defence secretary - who was given the Home Office post in Friday's Cabinet reshuffle - said he also had a lot to learn about other topics such as policing and victims' rights.
He said the challenge ahead was a daunting one, but added: "I've never shirked a challenge and I'm eager to get down to business."