Adam Jaylani, aged 21, of Orange Hill Road, Edgware, London appeared before St Albans Crown Court for sentencing on Thursday 18 March.
He was given a 21-month prison sentence and a 10-year restraining order in relation to the victim, which prohibits him from contacting her and her family, or visiting her address.
The defendant had previously pleaded guilty to actual bodily harm in relation to the assault on Monday 18 November 2019.
The court heard how Jaylani and the victim, who was 14 years old at the time, were known to one another.
They were sat in a stationary vehicle on Park Avenue when there was an altercation between them. Jaylani headbutted the victim before punching her in the mouth, causing facial bruising.
Detective Sergeant Dean Trollope, who oversaw the investigation, said: “Jaylani has shown himself to be a dangerous individual. The victim was terrified and she suffered significant injuries as a result of the vicious assault.
“Since the attack, the victim has been safeguarded by specially-trained officers and received help from Beacon, our victim support service. I hope Jaylani’s conviction and resulting restraining order have provided her with some closure, and she can now begin to move forward with her life.”