The 24-year-old was asked to play at left-back in our goalless draw with St Gallen on Wednesday night due to the absence of Andrea Dossena and Fabio Aurelio.
And Johnson showed some nice touches during his second-half bow.
Benitez told Liverpoolfc.tv: "I asked him to play on the left and he did really well. The team as a whole did well but everyone was watching Glen and he was good."
The boss took plenty of positives from the club's opening pre-season friendly despite the scoreline.
He said: "It was a good game, if a little open. There were opportunities for both teams so it's good for the fans. As a manager you are always pleased with a clean sheet and I was really pleased with some of the play in attack.
"We could see some really good play with some young players and everyone worked really hard.
"The young players did really well, though they'll need more time. Their workrate and attitude was the main thing."
Benitez also reserved a word for the hoards of Kopites who flooded into the AFG Arena.
"We have a massive following around the world and you'll see it in Asia next. It's fantastic," he said.