Mduduzi Bacede Mabuza and Mthandeni Dube were convicted by a high court in the capital, Mbabane, almost two years after the pair were arrested on accusations of having incited unrest.
"I find (the) accused... guilty of all charges preferred against them," Judge Mumcy Dlamini told the court.
The two, who had pleaded innocent to all counts, face up to 20 years in jail. A sentencing hearing is to take place "a date to be determined," said Dlamini.
Mabuza and Dube were held in July 2021 during protests calling for democratic reforms. The demonstrations were violently quashed by security forces, with dozens killed.
The verdict comes just months before parliamentary elections, which are due to be held on September 29.
The vote is unlikely to change the political landscape in the southern African nation of 1.2 million people, which has been ruled by King Mswati III since 1986. The king wields absolute power.
The convicted lawmakers had advocated for reforms to a complex system of non-party elections that effectively ensures Mswati faces no meaningful dissent.
Candidates cannot be affiliated to any political group under the constitution, which emphasises "individual merit" as the basis for selecting members of parliament and public officials.
Mswati can veto any legislation, appoints the prime minister and cabinet, and is constitutionally above the law.