Arlene Foster and Martin McGuinness wrote to Theresa May in August on their priorities for the UK's Brexit talks.
Mrs May's letter was obtained by the BBC's Nolan Show on Tuesday, prompting criticism it was not released in time for Stormont's Brexit debate on Monday.
Mrs Foster said she received the letter on Monday.
"It was released to the [assembly's] library at the same time as it was released to the media," she said.
"It's quite a significant response in so far as the prime minster has responded to all of the issues we raised with her."
In the letter, Mrs May restated the position that the government wants to see the maintenance of the Common Travel Area (CTA).
The CTA allows free movement of British and Irish citizens between the two islands.
In their letter to Theresa May, the first and deputy first ministers identified five key areas of concern regarding Brexit.
They were the border, trading costs, the energy market, drawdown of EU funding and treatment of the agri-food sector.
The first and deputy first ministers said Northern Ireland was unique as the only part of the UK that has a border with an EU member state.
The border must not become a catalyst for illegal activity or create an incentive for those who wish to undermine the peace process, they added.
In her letter, Mrs May says the future of the border is "an important priority for the UK as a whole."
She also says she recognises the "unique issues" raised by the Single Electricity Market and that resolving these will be a priority.
However some of the specifics of the Foster-McGuinness letter are not addressed.
For example, it stated that Northern Ireland businesses need to be able to retain access to "unskilled as well as highly skilled" migrant workers.
It also stated the need to "retain as far as possible, the ease with which we currently trade with EU member states."
Those issues of migration and market access will be key to the UK's Brexit negotiation with the EU and are not mentioned in Mrs May's letter.
While the UK as a whole voted to leave the European Union by 52% to 48%, 56% of people in Northern Ireland voted to remain.
In response to the Prime Minister's letter, Alliance MLA, Stephen Farry, said: "More concrete assurances are needed from the PM around Northern Ireland's status in the wake of any Brexit.
"This letter is just words, which do little to address the specific issues raised by the First Minister and Deputy First Minister."
The TUV leader Jim Allister said: "Not surprisingly, the letter from the Prime Minister to the joint First Ministers says nothing.
"Hence, perhaps, the reason why it had to be prised out of them and was not revealed for yesterday's debate."
Northern Ireland's first minister has rejected criticism over the way a letter from the prime minister concerning Brexit was made public.