Andy Ritchie's kids had already beaten County Tyrone and Toulouse before a goal from Martin Woods gave them a 1-0 win over famous Argentinean side River Plate.
The South American's were United's hardest opposition to date in the group, Leeds having to do some splendid defending throughout.
United now face Preston North End in the quarter-finals at Ballymoney Showgrounds. Aaron McConville scored twice as Preston put the brakes on Dynamo Kiev with a 3-1 success. Ben Kitchen added the third.
Details of group matches;
County Tyrone 1-0 (Aaron Lennon)
Toulouse 5-0 (Aaron Lennon 3, Kevin Smith, Jake Wright)
River Plate 1-0 (Martin Woods)