The Red Bull Racing boss was critical of the French manufacturer last year in a public war of words that almost saw the two parties end their long and successful collaboration.
This season however, following management changes and the arrival of renowned engineer Mario Illen, Red Bull and Renault have been the only team-engine combination to win a Grand Prix this year besides Mercedes.
"Obviously there have been some strong things said in the past – but that prompted action in Viry. Management changes were made and new personnel brought in, and that we are starting to see," Horner told the official Formula One website.
"It is a very cordial relationship now, and one that has been extended for another two years."
Despite the arrival of engine guru Illen, Horner believes there is more to Renault's resurgence than his signing.
"Well, Mario has a lot of experience – and that is an asset," he said. "But fundamentally the restructuring under Remy Taffin that has gone on in Viry – the wise recruitment of the right people from some of their rivals, and encouraging the talent that was always in Viry to step forward – made the change. Now we start to see the fruit of that."
Bahrain International Circuit - Winter testing