The lese-majeste law, which shields King Maha Vajiralongkorn and his close family from criticism, is often referred to as 112 in Thailand after the relevant section of the criminal code.
TLHR said the sentence was the longest handed down for royal defamation, beating the previous record of 43 years imposed on a woman in 2021.
Mongkol, who owns an online clothing store, was first arrested in 2021 during a protest demanding the release of political prisoners.
Youth-led demonstrations in 2020 and 2021 saw tens of thousands take to the streets, with many demanding changes to the strict lese-majeste laws.
TLHR said Mongkol would apply for an appeal against the sentence at the Supreme Court.
More than 250 activists have been charged under lese-majeste laws since the 2020 protest movement began, according to TLHR.
On Wednesday, one of the main protest leaders, lawyer Arnon Nampa, had his four-year lese-majeste sentence increased by another four years.