It is preparing for its first court hearing in November where it will demand that the Home Office applies the protection of the Children Act 1989 to children in prisons.
Staff lawyers have been preparing for the case since April and other charities including Barnardo's, Childline and Save the Children have added their support to the campaign.
"It's a bit of a baptism by fire, but we're prepared for the fight,
said Fran Russell, assistant director at the charity and head of the new legal department. "We believe that this new department has the capacity to make a tangible difference to the lives of many young people and children in prisons."
The department will handle judicial reviews, such as the current Children's Act case, which will work on changing policy and improving practice relating to youth justice law. It will also provide more straightforward representation for young people including services such as mitigating on sentencing.
Preliminary plans also include setting up a helpline for young people in custody, and offering second-tier advice services such as a youth defence representation.
"We looked at the system and recognised that young people in jail had no way of getting independent advice or legal services,
"This extension of our work is vital for the development of Howard League, and in a way forms the basis of everything we're trying to do."
Chris Callendar, legal officer for the youth justice law department, believes that this represents an important development for the sector.
"Before we launched this department, these kind of services simply didn't exist,
"Many charities have grave concerns about locking young people away in the current fashion, and believe that in this environment they are in desperate need of positive help and encouragement."
Howard League will attend a court hearing next month and will argue that children in young offenders' institutions should be offered the same care and protection by social services as those outside the legal system.
If the case is thrown out, the charity has said it will continue to fight to get the act amended.