The prize honours journalists working in difficult conditions in Asia, and is named after a crusading AFP reporter who died in 2007 at the age of 64 after a career covering wars and other historic events.
Mutita, 33, was recognised for her dogged efforts to record cases of alleged lese majeste for the online newspaper Prachatai, which publishes in Thai and English.
But Mutita refuses to allow cases to go unnoticed or unrecorded, pestering the courts for trial dates and documents to give publicity to cases that are otherwise easily buried.
She has followed cases including students performing an allegedly anti-monarchist play to a man accused of daubing defamatory graffiti in a toilet.
"She has been on the front line in the fight for freedom of expression in Thailand by persistently reporting on lese-majeste cases," said Andrea Giorgetta of the International Federation of Human Rights (FIDH).
Mutita's articles, which do not carry a byline, are a key source of information and data on lese-majeste sentences and convictions, said Human Rights Watch, hailing the "valuable work" done by the reporter.
In her reporting she strives to speak to those convicted and tell their stories.
And as punishments get increasingly severe - in August, a man was sentenced to 30 years and a mother of two young children got 28 years in prison for posting alleged defamatory messages online - her work gets harder.
The law, known by a shorthand of "112" from the relevant section of the criminal code, "is an issue that Thai society and media do not want to report on," Mutita told AFP.
A graduate of Thammasat University in Bangkok, which has a strong pro-democracy tradition, Mutita has been raising awareness of the bizarre nature of the trials: she often cannot even repeat the alleged offence without risking falling foul of the law herself.
With new restrictions imposed on prison visits by the military, Mutita cannot visit detainees as she did previously, to hear "their stories from their own mouth."
Kate Webb, after whom the prize is named, was one of AFP's finest correspondents. She earned a reputation as a fearless reporter but was equally known for her kindness and compassion and became a mentor to young Asian journalists.