Liverpool, with several changes to the team, laboured to a win that required extra time and 30 penalties in sudden death - in a game that saw two players score their first goals for Liverpool.
At opposite ends of the game, 17 year old debutant Jordan Rossiter sent a 30 yard shot into the Anfield Road goal, before substitute Suso scored with a Kop end goal in the second half of extra time.
Middlesbrough equalised twice - first from a free kick conceded by a Sakho handball, and then in the 122nd minute the men from Teeside earned a penalty which they dispatched past Mignolet.
Kolo Toure and Jose Enrique were handed their first starts of the season in a revamped back-line. Rossiter partnered Lallana and Lucas in the middle - with Sterling, Markovic and Lambert (named Captain) also named.
Rossiter scored the reds opening goal in the 10th minute, a ball rebounded to him 30 yards out and he fired a low drive into the net.
Middlesbrough were giving as good as they got, their good form in the Championship there for all to see.
A Sakho handball at the edge of the box allowed Boro to whip in a free kick that landed perfectly behind the back line and was dispatched from close range.
Balotelli was sent on, but it was another substitute Suso who in the second period of extra time put Liverpool back ahead.
But 2 minutes into injury time Toure conceded a late penalty, a shame after he produced a last ditch tackle in the 90 minutes to keep Liverpool in the game. Bamford stepped up and scored - sending the game into penalties.
Bamford then missed the first Boro penalty, but Sterling missed the reds 5th and the shootout was level at 4-4. On to sudden death.
Everyone else in the team took a penalty including the keepers to make it 10-10. And back around it went. Balotelli, Lucas, Lallana all scoring their second penalty, all matched by Boro. 13-13.
Suso stepped up and scored his second penalty and third goal of the game to make it 14-13. Finally Adomah missed a penalty and Liverpool ran out winners.
It was another below average performance from Liverpool but the reds go into the hat and must now rest ahead of the Merseyside derby on Saturday.