Ian Ogden, 27, was behind bars almost 150 miles away when the robberies were carried out on Teesside branches of the Market Cross Jewellers in Yarm and Middlesbrough.
Designer watches worth £637,000 were snatched in the raids, which were captured on CCTV.
The court was shown footage of both robberies in which display cabinets were smashed, and in one a handgun was produced.
Eight men from Manchester and Teesside admitted conspiracy to rob after the gang made mistakes in carrying out their plans.
Ogden, an inmate of Forest Bank Prison in Salford, had denied being involved.
But the prosecution said he used phones to link the Manchester and Teesside criminals in the planning and carrying out of the raids.
Richard Bennett, prosecuting, told the jury: "Ian Ogden may not have worn a mask or brandished a weapon but he was part of a criminal agreement to rob the two shops."
Ogden was convicted on Wednesday of two counts of conspiracy to rob following a trial at Teesside Crown Court.
Only one watch has been recovered from the haul of stolen items.
Ogden and the other eight men, who admitted the charges, will be sentenced at a two-day hearing on a date to be set.
Prisoner plotted armed raids from jail cell
03 March 2016
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A prisoner has been found guilty of plotting two armed smash-and-grab raids from his jail cell using smuggled mobile phones.
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