As there were dog cages in the back, one girl climbed into the passenger seat and the other one sat in the passenger footwell.
"While driving with the girls in his car, the defendant learned that they ran away from a CYFS family home."
He also learned they were short of money, Judge Cameron said.
About 20 minutes south of Whanganui, Johnston pulled the car over and the girl in the footwell moved into one of the dog cages.
Johnston then offered to pay the girls for sex, and both refused.
He then offered to pay them to perform oral sex on him, but they again refused.
Johnston performed an indecent act while sitting in the driver's seat, Judge Cameron said.
Johnston told the girls to watch him and said he'd give them $20 if they did.
One of the victims began to cry.
He dropped the 15-year-old victim at a service station and headed back to Whanganui with the other victim.
He began asking the girl "sexualised questions" including asking if she liked sex.
Johnston drove the girl to her grandparents' house before returning home to Foxton, ending what had been a 400km round trip.
He pleaded guilty to two counts of performing an indecent act on a female aged 12-16, and two counts of encouraging an under-18-year-old to provide sexual services.
Judge Cameron said Johnston had previous convictions from 1982 to 2013, including a previous conviction for encouraging an under-18-year-old to provide commercial sexual services.
He received a sentence of 100 hours community work for the crime back in 2013.
According to victim impact statements, both of his new victims felt angry with Johnston.
One said she had issues of trust with people as a result of the incident, while the other had "lost confidence" and "at one point had thoughts of self-harm".
Johnston was assessed as being at medium risk of harm to others, and in his pre-sentence report attempted to "minimise" his actions by saying the girls were encouraging his behaviour.
"I don't accept that for one moment, and the summary suggests quite the contrary," Judge Cameron said.
He sentenced Johnston to 12 months in prison, with six months of release conditions.
The judge gave him a strike warning under the three strikes law.