He got hooked on management playing the computer game Football Manager in his teens before taking over at the Wessex League One club earlier this year.
Romsey had just been relegated and needed a complete overhaul - so Phillips, who was the reserves’ assistant manager, was promoted.
He began his coaching career at just 16 with fellow Hampshire club Lyndhurst and already has his FA Level One and Level Two badges.
Despite managing players over ten years older than him - club captain and centre-half Ben Andrews is 33 - he still has the respect of the squad.
He said: "It's been a difficult few seasons for the club and that was the reason we wanted to change things - we only kept hold of two players.
"It's my job to try and get the team back up at the first attempt so there are a few nerves but I'm excited.
"We've done a massive rebuilding job and recruited around 30 new players who all can't wait to get out and play in the FA Cup.
"The club made the first round last year - we have four or five games to reach that this year but I would like to think we can match that."
Phillips gained his coaching badges doing a course run by Saints while studying at Totton College, Hants.
He has continued to improve himself as a coach completing the FA Level Two course and will next month start his Uefa B Licence.
That will enable him to be as qualified as managers and coaches in the Football League.
He was first offered the position of assistant reserve-team manager at the Bypass Ground last season and enjoyed a successful spell in charge.
Impressive runs in the Southampton Senior Cup and Hampshire Intermediate Cup followed, and in April he took over from Carl Bennett and John McFarlane.
He's now turned his back on a career as a goalkeeper - citing his height of 5ft10in as not tall enough to make a success of himself in the game.
The custom vehicle fitter said: "I had the ability but I didn't think I could progress in the game so I took up coaching.
"I've always been interested in management and the tactics of the game - ever since I got my first Football Manager computer game.
"I never did take them into the Champions League - the new version of the game had come out by them."
Phillips, who lives with his mother Viv and stepfather Maurice, is expecting at least 150 people to watch this Saturday's clash.
He added: "I've heard I'm the youngest in the competition's history so that's something to be proud of.
"We have youth teams at the club so hopefully lots of those younger boys turn out with their dads."
At 33, Stan Cullis led Wolves to beat Leicester in 1949 - making him the youngest ever FA Cup winning boss.
FA Spokesman David Barber said: "I’ve been our historian for 30 or more years and I've never heard of a cup manager aged 22.
"James would be the youngest by my reckoning.
"Last season we had an assistant referee in one cup match who was 16 -Rugby Town v Norton United in the Preliminary Round!"