USA's draw later in the evening against China PR, leaves Germany in a position to lock up top spot in the group with just a point against the Stars and Stripes in Monday's final round.
Germany had the better of the contest but the Africans battled hard throughout, only for Magull to profit from a perfect Dzsenifer Marozsan pass and score the all-important goal just eight minutes after entering the fray.
Ghana made three changes to the side which were beaten 4-0 by USA in the opener, and it was a much-improved performance from the Black Princesses.
A Luisa Wensing header forced a great save from Ghana goalkeeper Patricia Mantey early on, while at the other end, Florence Dadson’s effort required an equally impressive stop from Germany custodian Laura Benkarth.
The champions soon started to assert their authority and Nicole Rolser twice, and Lena Lotzen, came close to breaking the deadlock before the interval.
Mantey was inspired form after the break making several important stops, notably denying Leonie Maier, however the goalkeeper’s heroics were to no avail thanks to Magull’s late intervention.