Led from the front by David Warner's 178 runs from 133 balls, which included 24 boundaries, Australia posted 417-6 from their 50 overs, with Warner aided by 95 from Steven Smith and a rapid 88 off just 39 balls by Glenn Maxwell.
Afghanistan, who won the toss and chose to field first, didn't stand a chance of chasing the mammoth total down, but still put in a respectable batting display in what is their first World Cup showing.
Johnson took a game-high four wickets and also removed Afghanistan's highest scorer Nawroz Mangal (33), with the minnows losing all 10 wickets for 142 in the 38th over.
After winning one, losing one and having one of their first three games abandoned, Australia now sit third in Pool A on five points behind New Zealand and Sri Lanka. Afghanistan remain on two points with one win and three defeats to their name.