Leckie, who joined the Bundesliga side from Ingolstadt, was in the starting XI on a day Sadio Mane made his return from injury and Philippe Coutinho captained Liverpool despite continued reports he is nearing a move to Barcelona.
Reports in Spain have suggested that Brazil star Coutinho is on Barca's radar should his international team-mate Neymar complete a world-record move to Paris Saint-Germain.
However, the skilful playmaker travelled to Berlin to wear the captain's armband and was influential in Liverpool's third goal that completed a well-deserved win.
Coutinho played a sublime ball to Mohamed Salah, who provided an equally exquisite finish for his third pre-season goal since joining from Roma.
Liverpool were already 2-0 up at half-time as close-season signing Dominic Solanke, whose inclusion was a late one due to hamstring tightness for Divock Origi, headed home before Georginio Wijnaldum stroked in the second.
In another boost for Jurgen Klopp, Mane – who set up Wijnaldum's goal – came through 45 minutes in his first outing since undergoing knee surgery in April, while Andrew Robertson made his debut after joining from Hull City in the second half.
Hertha threatened early with Mitchell Weiser scuffing wide after skinning Alberto Moreno, while Vladimir Darida stung Simon Mignolet's palms from distance.
But Liverpool settled and had the lead in the 15th minute. Adam Lallana dinked a lovely ball into the area and Solanke peeled off his marker to loop his header over Hertha goalkeeper Rune Jarstein.
Mane showed great persistence to charge down two challenges before blazing a volley over and the Senegal winger had legitimate claims for a penalty turned down when tripped by Karim Rekik.
It was 2-0 seven minutes before the break, though. Solanke fed Mane in the box and he in turn laid off for Wijnaldum to coolly guide a 20-yard effort into the bottom-left corner.
Robertson played a teasing ball across the face of goal from the left that Salah should have done better with against an unguarded net.
But Salah made amends in fine fashion in the 61st minute. Coutinho weighted a wonderful ball into his path, and he brilliantly dinked his effort over Jarstein.
Roberto Firmino almost added a fourth with a chipped attempt, while Daniel Sturridge had an effort palmed away as Liverpool coasted through the closing stages.