The 23-year-old scored the winner for the Valiants in their 1-0 victory at Stafford Rangers this afternoon.
Fellow trialists Stuart Tomlinson and Kris Taylor also played, but Thornton was the most impressive performer on show after being given an attacking midfield role.
Thornton, from the Republic of Ireland, played for Vale boss Micky Adams at Coventry, and again on loan at Brighton last season.
In June, Thornton was convicted of drink driving. He was released by the Sky Blues earlier this month. He had a year left on his contract, but Coventry announced he had left by mutual consent.
He was given the full 90 minutes to shine this afternoon as the Valiants were given a decent work out at Marston Road.
He looked well short of match fitness, but demonstrated enough ability to show why Adams has given him a chance.
The manager split his squad so Vale could also play a behind-closed-doors friendly at Crewe, but he was at Stafford to see Thornton quickly into the action.
The midfielder's inswinging free kick found Gareth Owen at the back post inside two minutes but the captain missed his kick.
But Tomlinson came to Vale's rescue on six minutes, saving from Nick Wellecombe after the striker had been played through by former Vale youth player Jake Moult.
Wellecombe then unwittingly deflected Bobby Wilson's close-range header wide of the Vale goal, before Tomlinson saved from Ben Mills.
But Stafford keeper Lee Evans also shone on 33 minutes as he blocked Doug Loft's powerful 16-yard drive.
Luke Morgan's 20-yard strike was well held by Tomlinson on 36 minutes before Mills fired wide after wriggling free on 42 minutes.
But Richards volleyed wide at the near post in first-half injury time after he connected with Danny Glover's cross.
Vale were fortunate to survive on 48 minutes when Andy Owens rose highest from a corner, but planted his header against the bar.
But Richards's 53rd-minute header from Dodds's cross was saved by Evans, before the keeper made an even better stop to push away Richards's 20-yard strike a minute later.
Richards curled a free kick just wide, but Daryl Proffitt's powerful strike was held at the second attempt by Martin in an evenly contested half.
Thornton's effort was blocked by Evans after the trailist jinked free in the area. But the trialist grabbed the winner on 86 minutes when substitute Sam Morsy's shot found Thornton in the area. He controlled the ball, and showed excellent technique to sweep it left-footed past Evans.
STAFFORD: Evans, Francis, Vauls, Thorley, Wilson, Owens (Page, 59), Morgan (Hulme, 71), Moult (Rodgers, 84), Wellecombe, Mills (Miller, 76), Proffitt (Hill, 71).
PORT VALE: Tomlinson (Martin, 45), Yates, Owen, Collins, K Taylor, Griffith, Thornton, Loft, Dodds, Glover (Morsy,71) Richards.
Subs: Beeston, Martin.
Attendance: 608