The coalition minister is currently fighting an internal battle over future welfare restrictions after the next general election.
He has faced heavy criticism from campaigners over the extent of the cuts brought in to date and controversial issues like the so-called "bedroom tax".
But yesterday Mr Duncan Smith said: "We are delivering, and it is changing our country for the better."
He added: "Immigration into the UK has been a supply and demand issue.
"Businesses needed the labour and because of the way our benefit system was constructed, too few of the economically inactive took the jobs on offer.
"When we took office, there were nearly five million people on out of work benefits.
"It was clear to me that in large part this situation was the product of a dysfunctional welfare system that often trapped those it was supposed to help in cycles of worklessness and dependency.
"My one aim as Work and Pensions Secretary has been to change this culture - and everything we have done, every programme we have introduced, has been about supporting everyone who is able to into work."
He also argued that the reforms, which include the heavily criticised "fitness to work" tests, have led to a significant drop in unemployment.
His department has also faced questions over the implementation of Mr Duncan Smith's much-vaunted "universal credit", which is designed as a way to ensure that work always pays.
The scheme has been beset by problems and faced a series of delays.
Last week, Ben Broadbent, who is the deputy governor of the Bank of England, said Mr Duncan-Smith's benefit cuts had encouraged more people back into work.