The 22-year-old American scored his first goal in English football in the Cobblers' 2-1 win against Cheltenham.
Sampson told BBC Radio Northampton: "He's worked ever so hard and improved a lot since Christmas.
"It's not easy to come such a distance and try your luck in the English game. It's a totally different environment. He deserves quite a few plaudits."
But Sampson conceded there was a chance Harris could have left the club in January. Having joined in August, the striker had only made three appearances prior to the Cheltenham match.
"It was 50-50. We saw some progress in his play and his attitude, and that's why we kept him on," Sampson said.
Source: BBC Sport
Source: BBC Sport