Robbie Brady scored within a minute of coming off the bench as Hull needed extra time to overcome Leyton Orient.
Orient, who had won all five of their previous matches this season, dominated at Brisbane Road and should have led through David Mooney's header.
Orient continued to create the better openings, although Hull's David Meyler had a header cleared off the line.
Former Manchester United forward Robbie Brady managed just four goals in the Championship last season but has already scored two in his first three appearances of the season
In the second period of extra time Brady pounced on Liam Rosenior's ball to send Hull into the third round.
With manager Steve Bruce making 11 changes, the Premier League side were not as fluent as Orient who twice went close to taking the lead when Romain Vincelot shot wide from Lloyd James' corner and Kevin Lisbie headed over.
Mooney squandered the best chance of the opening period when he headed wide from a Dean Cox cross.
The Premier League side finally threatened in the first minute after half time when Meyler met Cameron Stewart's corner and Elliot Omozusi hacked off the line. Cox came close to grabbing victory for Orient in normal time but curled his shot over the bar from the edge of the area.
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