The news - unusually delivered by two ministers - comes two days before the start of the pre-referendum period, when Scottish Government announcements are severely restricted.
They said work on a five-mile, £50 million section of the road between Kincraig and Dalraddy would now start next summer thanks to good progress with preparatory work, and savings with the Queensferry Crossing project, which had freed up funds.
A total of 80 miles of single carriageway between Perth and Inverness are to be dualled by 2025, with ministers pledging to complete half by 2022.
Mr Brown today again underlined the urgency of the project.
He said: “Dualling the A9 between Perth and Inverness is a huge challenge – some 80 miles of upgraded road, but we are determined to make early wins.
“I am pressing everyone working on the A9 – both short and long term programmes, to get on and deliver early benefits to users of the road as soon as is practicably possible.
“As the Queensferry Crossing rises out of the Forth estuary, we are anticipating more savings being realised next year from the project’s contingencies, allowing us to unlock investment for the A9 dualling scheme now.”
Mr Swinney stressed the employment benefits of the scheme.
He said: “The ground investigation work alone under way here at Kincraig is supporting over 30 jobs, with more to come when full construction begins next year.
“Communities and businesses here and along the length of the route can look forward to feeling all of the benefits of this investment as we press ahead in delivering the full dualling programme.”
However, Chief Secretary to the Treasury Danny Alexander said Highlanders would treat the news with derision since the Scottish Government had committed to dual the road in 2007.
The Inverness, Nairn, Badenoch and Strathspey Liberal Democrat MP said: “Regular uses of the A9 have had to endure seven wasted years of a SNP government that has focused on Central Belt priorities while turning a deaf ear to Highland concerns.
“So far, not a single metre of the A9 has been dualled on their watch.”