The conviction of Birmingham man Zakaria Mohammed is the first time a conviction has been secured for child trafficking offences related to county lines under the Modern Slavery Act (2015).
The legislation introduced tougher sentences for those who traffick children and exploit them for criminal purposes, including sexual exploitation and drug networks.
County lines involves gangs from urban areas who exploit children to set up drug distribution networks to rural areas, market towns and coastal areas.
Gangs and drug dealers often use violence and intimidation to set up and maintain such networks. This also includes forcing vulnerable people from their homes to set up drug dealing bases, a practice known as "cuckooing".
The most recent national assessment of county lines, by the National Crime Agency, suggests there are more than 1,000 such networks in operation.
Mohammed, aged 21, was sentenced to 14 years in prison at Birmingham Crown Court this week after admitting running a drug dealing operation and trafficking two 15-year-old boys and a 14-year-old girl.
His conviction follows a West Midlands and Lincolnshire Police investigation after the two boys were reported missing from their home in Birmingham and later found in Lincoln earlier this year in a flat where knives and a bundle of cash were discovered.
The investigation revealed that Mohammed had been making regular trips to the flat from Birmingham and a phone used to run the drugs line was found in his car as well as evidence linking him to the missing children.
Mohammed refused to talk to detectives during interview but later admitted four counts of possessing drugs with intent to supply and five counts of human trafficking.
"This is a hugely significant conviction for West Midlands Police and law enforcement as a whole across the UK," said investigating officer detective constable Max Gebhard.
"It shows that we can go after county lines offenders not just for drug supply but also under trafficking legislation due to them exploiting children. And that means stiffer custodial sentences for offenders."
Last month the Home Office launched the £3.6m National County Lines Co-ordination Centre, to tackle county lines and develop national intelligence of the complexity and scale of the problem.
The Home Office has said that there are more than 200 active county lines investigations under way.
Evidence released this week from a Home Office-funded pilot to tackle county lines crime found that specialist support from a youth charity was able to dramatically reduce the number of children who go missing.
"This conviction under the Modern Slavery Act sends out a clear signal that children under the control of so-called county lines drugs gangs are not criminals, but victims of criminal exploitation and trafficking," said The Children's Society chief executive Matthew Reed.
"These gangs groom children with drugs and alcohol or promises of status and wealth, and control them using terrifying threats, violence and sexual abuse.
"We work with children who have gone missing from home or care, sometimes for weeks on end, at the mercy of gangs who see these vulnerable children as pawns in their crimes.
"That's why when a child is reported missing from home or care they should be assessed to see if they're at risk of being exploited by these gangs and professionals must get better at spotting the signs and ensuring these children get help at a much earlier stage."