IF's UK-based staff will also join Hotwire in the UK, under Brendon Craigie, group CEO.
Craigie told PRWeek: 'We have made our name as an agency on a local level in different countries, then at a regional level. This is a massive opportunity to shake up the US market.'
Hotwire US will operate from San Francisco and New York and will start with IF's 35 clients already on the books.
Craigie hopes that the US operation will grow to 20 people by the end of the financial year and that US growth will enable Hotwire to double in size over the next five years.
He added: 'The original inspiration behind Hotwire was to be able to provide US companies with the capability to reach all of the major European markets. Ten years on, we can now offer that service in both directions.'
Fox said IF had grown US fee income to US$750k-$1m in the past 12 months.
She added that the move had been driven by client demand: 'Big global tech and mobile companies are looking for agencies that can work with them both within and outside the US. There is much more demand for that one-stop-shop model again.'
IF's clients are predominately in the tech and mobile comms space, including Silicon Valley Bank, Intel Capital, InMobi and Octopus.