But critics say it risks further reducing social housing stock, with housing charity Shelter describing the plan as "reckless".
Ministry of Housing, Communities and Local Government figures showed more than 1,500 homes were privatised through the Right to Buy scheme in the county between 2012-13 and 2020-21.
Councils which raked in more than £90 million from the sales are expected to replace these homes on a one-for-one basis.
Yet just 565 replacements were acquired or began construction in the area over this period.
In the former Northampton Borough Council area 907 homes were sold but only 359 replacements were in pipeline.
In Kettering, 219 council homes were sold with just 70 replacements while in Corby another 392 privatised council homes were replaced by just 136 properties.
Right to Buy was introduced in 1980 to help council and housing association tenants buy their home, at a discount of up to £87,200 outside London.
Mr Johnson sought to move on from the recent confidence vote and Partygate scandal with a major policy speech in Blackpool, focusing on addressing the cost-of-living crisis.
The PM said ministers would seek to “finish the right-to-own reforms Margaret Thatcher began in the 1980s” and extending the Right to Buy scheme to those “trapped” in their housing association homes.
He criticised some associations for showing “scandalous indifference” to tenants and pledged a one-for-one replacement of each property sold to prevent the housing stock dwindling.
Across England, 96,100 homes have been sold through Right to Buy since 2012-13 with just 32,900 replacements.
Alicia Walker, head of policy, research and campaigns at homeless charity Centrepoint, said: "Extending the Right to Buy to housing associations risks further eroding the stock of social housing, which many young people desperately need.
"Promises of like-for-like replacements have been made before but not been followed through.”
Mr Johnson also announced a change to welfare rules so that the 1.5 million people who are in work but also on housing benefit will have the choice to use their benefit towards a mortgage.
Housing and homeless charity Shelter described the PM’s policies as "baffling, unworkable, and a dangerous gimmick."
Polly Neate, chief executive, said: “Hatching reckless plans to extend Right to Buy will put our rapidly shrinking supply of social homes at even greater risk.
“The maths doesn’t add up. Why try to sell off what little truly affordable housing is left – at great expense – when homelessness is rising and over a million households are stuck on the waiting list?
“The government needs to stop wasting time on failed policies of the past and start building more of the secure social homes this country needs.”
Northampton Council collected £54.4 million through the Right to Buy scheme over nine years, according to the figures while Kettering council banked £13.9 million and Corby council collected £22.1 million.