The attack on 7 March 2014 caused no deaths or injuries but was one of a number of tit-for-tat assaults during months of often violent street protests that left at least 28 dead and hundreds injured.
The trial at Bangkok's Criminal Court heard the two men belonged to the United front of Democracy against Dictatorship (UDD) -- the official name for the "Red Shirt" movement loyal to ousted premiers Thaksin Shinawatra and his sister Yingluck.
Sentencing the two men Friday, a judge said "witnesses gave concrete evidence" against the pair, adding that their life sentences had been reduced to 43 years and four months because they "confessed during their interrogations".
Charges against the pair included the possession of unregistered weapons and premeditated attempted murder.
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The M79 grenade was fired at a crowd of anti-government protesters near the city's famous Chatuchak market.
Last year's protests were part of Thailand's long-running political conflict that broadly pits a Bangkok-based middle class and royalist elite, backed by parts of the military and judiciary, against rural and working-class voters loyal to the Shinawatra clan.
The ongoing unrest and inability of Yingluck's government to function led to army chief Prayut Chan-O-Cha instigating a coup, the second military takeover in under a decade.
The military said the coup was necessary to restore order and end the country's cycle of violent street protests.
Since seizing power Thailand's generals have largely succeeded in curbing public dissent by stamping down on basic rights.
While Prayut, now prime minister, has promised a return to democracy, an election date has repeatedly slipped.
Critics say the coup was the latest move by the country's elite to grab power and prevent democracy from taking root in the kingdom.