Law reform and human rights charity Justice welcomed the fall, but said more must be done to tackle inequality in the youth justice system so all children are treated with care and dignity.
Ministry of Justice data shows 183 under 18 year olds handled by Devon and Cornwall Police entered the criminal justice system for the first time in the year to September – a 79% decline from 870 a decade prior.
Of the juveniles entering the system last year, 44 were convicted of a crime (24%) and the rest were cautioned or reprimanded.
And where gender was recorded, 138 juveniles were male (75%) and 45 were female (25%).
Across England and Wales, over 8,300 young people entered the criminal justice system for the first time last year. It was down slightly from 8,800 the year before and a fall of 74% from 32,000 in 2011-12.
About 48% of young first-time entrants to the justice system were convicted of a crime.
Tyrone Steele, criminal lawyer at Justice, said the declining number of children entering the system is welcome.
Mr Steele added: "This has not benefitted all communities equally. Ethnic minority children remain vastly overrepresented, still making up over half of those in the youth custody."
He said it is clear more must be done to tackle racial injustice in the youth criminal justice system.
"This means rolling back on expanded stop and search powers and focusing instead on tried and tested diversion programmes, which serve to ensure that all children are treated with the care and dignity that they deserve, providing vital support rather than damaging criminalisation,” he added.
The figures show five children in Devon and Cornwall who entered the criminal justice system for the first time were from an ethnic minority background – 3% of juvenile offenders.
Nationally, 20% of juvenile first-time entrants were black, Asian, or Middle Eastern.
A National Police Chiefs’ Council spokesperson said forces across the two nations work hard to avoid unnecessarily criminalising children and young people.
“Our child-centred approach means police will consider a range of options to avoid criminalising young people for their first minor offences," they added.
"Using out of court disposals and restorative justice discourages further offending whilst ensuring an admission of responsibility is sought and damage caused can be remedied.”