Denton captained the under-21s in their 3-3 draw with Bolton Wanderers on Monday, standing in for the suspended Lewie Coyle.
The left-back signed a new contract in the summer and told the club website he aimed to push for a first team place this coming season, to challenge for Charlie Taylor's spot.
Hart is impressed with his progress, but explained to BBC Radio Leeds that he was not initially enamoured with the defender, having heard glowing reports about him upon his return to the club.
He said: "Since I came in, people talked about him, he had a reputation... I didn't see it in the first two or three games, but we've done a bit of work with him to get on the front foot and not allow the winger any breathing space.
"The last two games he's been absolutely excellent. He's listened, he's learned, he's seen the value, and he's played the best two games I've seen from him and consequently whoever he's been marking has been uncomfortable."
Hart added that he felt Denton lived up to his captaincy and displayed leadership qualities in the entertaining clash with Bolton, which saw Tommaso Bianchi return to action following a spell on the sidelines with a knee injury.
For Denton, breaking through into the first team will not be easy, but with Charlie Taylor out for a short period with injury, there is a possibility his form sees him called up to the bench as cover for Gaetano Berardi, who deputised at left-back over the weekend.