He met Northern Ireland's newly-appointed leaders for talks set to focus on how much the Government will stump up to support the return of devolution.
But he refused to confirm whether a reported £2 billion finance package would be provided to help the powersharing deal, saying its "not just about money".
"It is about leadership, it is about getting the public the services they need, particularly in healthcare," he said, but he added how the UK government would "do everything we can to support".
He was speaking after talking with Northern Ireland's First Minister Arlene Foster, from the DUP, and her deputy, Sinn Fein's Michelle O'Neill.
The meeting came after a three years impasse of no devolved government in Northern Ireland was ended by when a breakthrough was made on Friday.
The PM said: "I just want to say how grateful I am to all the parties, to everybody here in Northern Ireland, for the way they have compromised the way they have worked together to get Stormont up and running once again.
"It's shown a willingness to trust each other and to set aside differences and I think it's absolutely commendable and wonderful to see."
Ahead of Mr Johnson’s arrival, a Stormont minister said he expected the government to deliver at least £2 billion to support the powersharing deal
DUP agriculture minister Edwin Poots warned that the money could come with “strings attached”, potentially by way of a commitment from the executive to raise extra revenue through the introduction of water charges or a hike in rates bills.
The UK government made a series of financial promises as part of efforts to get the “New Decade, New Approach” agreement over the line.
Mr Smith, who helped broker the deal, pledged major investment to alleviate problems in the region’s struggling public services, but declined to confirm the sums involved until a devolution was restored.
Ministers are now keen to get those figures nailed down, with Mr Poots telling BBC Radio Ulster he anticipates the amount to be in the billions.
"He said he didn’t want to make any promise on a figure and I said all you have to say is ‘billions’ and, of course, that would be at least two billion," he said.
However, a Number 10 source described the £2bn figure as “just speculation”.
The Irish government has also made financial pledges within the agreement to honour commitments to part-fund some north/south projects, such as the A5 dual carriageway and a redevelopment of the Ulster canal system.
Mr Johnson said the agreement to restore power-sharing meant the devolved government would be able to deliver "much needed reforms to public services".
Ministers will need to hit the ground running to tackle a host of acute problems facing a public sector that has been floundering amid the 36-month governance vacuum.
A priority for the new devolved government will be to address an industrial dispute in the health service that has seen nurses take strike action on three occasions in the last month.
Due to a lack of legislature in Northern Ireland, NHS nurses there have ended up being paid less than nurses in the rest of the UK and want a pay rise.
Under the terms of the deal, the new Executive will also take action to reduce spiralling hospital waiting lists; extend mitigation payments for benefit claimants hit by welfare reforms; increase the number of police officers on the beat; and resolve an industrial dispute involving teachers.
The last DUP/Sinn Fein-led coalition government collapsed in January 2017 over a row about a botched green energy scheme.
That row subsequently widened to take in more traditional wrangles on matters such as the Irish language and the thorny legacy of the Troubles.
Mr Johnson said discussions at Stormont would focus on how the executive intended to take forward “critical reforms” to public services.
“This is an historic time for the people of Northern Ireland,” he said ahead of the visit.
“After three years, Stormont is open for business again with an executive who can now move forward with improving people’s lives and delivering for all communities in Northern Ireland.
“I look forward to meeting with the new executive and hearing about their plans for the future – including driving forward much-needed reforms to public services and resolving the current health strike.
“The next decade will be an incredible time of opportunity for Northern Ireland and the whole of the United Kingdom as we come together to unleash the potential of our four nations.”