Portsmouth paid £4m for Johnson, who emerged as Benitez's prime target after having an outstanding season at Fratton Park last season.
"It was down to the player whether he chose us or City or Chelsea and he chose us," Benitez told the Liverpool website. "It was very clear from talking to him that he wanted to play for Liverpool.
"He was important at Chelsea and in the last year at Portsmouth he has improved and is much better.
"One of the main things is his mentality - it's really good. He is a winner. I was impressed with the conversations I had with him. He wants to improve."
Johnson signed a new four-year deal at Portsmouth in January, but club officials accepted they could not stand in his way if he was offered the opportunity to move to a club playing in the Champions League.
"We were thinking about bringing in players with quality - that's the first thing," added Benitez.
We could buy one more player without selling anyone. We now have to wait
Liverpool boss Rafael Benitez
"He is also English and that is important for the Champions League list. We were thinking about improving the team and Johnson is a very good player who will do that."
Johnson's arrival could now signal the departure of Spanish right-back Alvaro Arbeloa, with Real Madrid being linked with an £8m bid for the 26-year-old.
"Arbeloa was a good player for us last year," said Benitez.
"The thing is, he has one year left on his contract. We have offered him an extension and we're talking.
"He is our player but if there is a good offer we have to consider it. We have two or three clubs asking about him. He would like to stay but he knows the competition will be hard."
He added: "We could buy one more player without selling anyone. We now have to wait."
Italian left-back Andrea Dossena could also leave Anfield, while Reds midfielder Sebastien Leto is on the verge of a move to Greek side Panathinaikos.
The Argentine, who joined Liverpool from Lanus for around £2m in 2007, was on loan at Olympiacos last season, but he has grown impatient waiting for them to make their move.
"I think that the possibilities to play at Panathinaikos next season right now are more than 90%," said the winger.
"I have been waiting for something from Olympiacos all these weeks but they didn't make a move to buy me so far."
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