England were coasting in their reply as long as Jonny Bairstow (105 in 59 balls) was in charge, but Kyle Coetzer's men held their nerves to register their maiden victory over the opposition at Grange Cricket Club in Edinburgh.
Scotland's Safyaan Sharif sparked a pitch invasion when he trapped Mark Wood lbw with an in-swinging yorker in the 49th over just when England looked like they might avoid an infamous defeat that ranks alongside their loss to Ireland at the 2011 World Cup.
But they could have done with debutant spinner Hamza Tahir - cousin of Scottish record limited-overs wicket-taker Majid Haq - finding his feet at worldwide level a bit quicker.
Former Durham and Warwickshire batsman MacLeod said Scotland's first global cricket win over England was a "massive statement".
Eoin Morgan's team have played a fearless brand of cricket over the last three years, with the 2019 World Cup fixed in their crosshairs, but have also crashed and burned on more than one occasion in that time.
MacLeod, whose unbeaten 140 off 94 balls was integral to the historic humbling of England in Edinburgh on Sunday, will be joined at the 2017 Blast quarter-finalists by his national coach Grant Bradburn, who will assist fellow New Zealander John Wright for the entire tournament.
"I'm absolutely happy with that, it doesn't bother me", Morgan said.
Agar said that Australia's squad had a "nice refreshed" feel to it and was particularly excited at the prospect of tall quick bowler Billy Stanlake taking on the powerful England batting line-up. "Anything above 350 is a big total and we know how risky this England side can be, so now it's up to us to bowl well". Coetzer and George Munsey were off the blocks swiftly, showing an eagerness to surmount another big challenge.
Test skipper Joe Root was carelessly run out and England then lost Morgan and Alex Hales off successive deliveries to be 245 for five. Mark Watt finished on 3-55 off 10 overs.
"I can't talk about it any more to be honest, we've talked about what's acceptable from our team and what people are saying from outside our team doesn't really matter", Paine said. "I think it's going to be a good series".
Alasdair Evans of Scotland takes the wicket of David Willy.