The Rowlands Castle talent defeated Yorkshire’s Alex Fitzpatrick 3/2 on the 34th hole of the final on Sunday.
He then toasted the biggest moment in his fledgling career, while saluting the assistance of friends and mentor Scott Gregory.
Mckenzie, 23, said: ‘It’s unbelievable.
‘To win one of these big match play events is a dream come true.
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‘For the sake of 90 minutes football, it’s not worth a player being off work for a year or ending up in a wheelchair’ – Hampshire grassroots football clubs postpone matches due to the heatwave
‘I’m very close with Scott (Gregory).
‘We have the same coach and we have been pushing each other on.
‘I’ve seen his successes. I have watched him grow as a player and go on to great things.
‘He was giving me advice during the week and telling me to bring it home, which he couldn’t!’
Corhampton’s Gregory, who recently recorded his maiden victory as a professional in the Penina Classic IV, was a losing finalist in the Spanish Amateur in 2016.
Mckenzie, however, follows in the footsteps of six othere English winners of the prestigious title in the past 15 years.
Those champions include Danny Willett, who went on to win The Masters.
Sunday’s triumph completed a remarkable turnaround for Mckenzie, who just scraped into the matchplay stages in the 32nd and final spot after an anxious wait at the end of the 36-hole qualifying.
He produced flying form from there on.
The Hampshire star defeated top qualifier and England international Jake Burnage by one hole in the first round and gained steadily in confidence.
Mckenzie added: ‘I wanted a decent result to start the season and felt getting to the last 16 would be acceptable.
‘After that I just freed up and played it one game at a time. And it became, if not easier, less pressured.’
Mckenzie’s closest shave was in the quarter-finals where he had to go to the 20th.
But he won his semi-final 5/4 and soon took charge in the final, leading by three holes after the first 18.
He pushed further ahead after lunch, getting to six-up after 22 holes.
Fitzpatrick launched a fightback and after 32 holes only trailed by two.
However, Mckenzie won the 33rd to return to three-up and after the players halved the 34th the title was his.
The Rowlands Castle player is the latest amateur success story in Hampshire after Harry Ellis (2017) and Gregory (2016) won the past two British Amateur crowns.
Mckenzie, who helped Hampshire win the English County Championship last season, added: ‘We are one big team. We all support each other.
‘I couldn’t hope for a better band of lads to push for me this week – as I have pushed for them in other tournaments.’