Joao Morelli scored four and Mark Kitching an excellent solo goal as Boro ran out comfortable winners, although credit to Thornaby who, with ex Boro Academy players Kieran Edwards, Rocky Andrews and Theo Furness in their squad, stuck to their task and never gave up.
The opening goal came on 14 minutes when Nathan McGinley crossed low from the left and Morelli was left with a simple task to tap in at the back post.
Thornaby goalkeeper Michael Duff produced several smart saves to keep Boro at bay and it was the home side who went close when right-back Stephen Morrison drilled in low from the right, a delivery that was just out of the reach of centre forward Gary Mitchell.
Boro went two up on 23 when a shot from Arnel Jakupovic was parried in to the path of Morelli who tucked in from close range.
Thornaby pulled one back six minutes later when Thomas Bligh shot low from the right. It was an awkward one to deal with and Mitchell was on hand to capitalise as he followed up to tap home.
The best goal of the night came three minutes from half-time when Morelli sent a dipping volley into Duff's bottom right-hand corner from 20 yards, then Jakupovic hit the goalkeeper's right-hand post with the final shot of the half.
It took just 64 seconds of the second half for Boro to score another as a driving run from Joe Wheatley had the home side on the back foot and Morelli drilled home low from 20 yards.
Less than a minute later Jakupovic should have made it five when he rounded the goalkeeper but centre-half Stuart Browne cleared off the line.
Richard Walker sent a low left-footed shot just wide from the edge of the penalty area, Mitchell curled just over from 20 yards and Duff was forced in to another save, this from Tom McAloon.
It wasn't until the 83rd minute that Boro added another as Mark Kitching danced his way through before sending a low shot beyond Duff.
Thornaby: Duff; Morrison, Robinson, Bythway, Browne, Edwards (Capt.), Bligh (Butterworth 60), Andrews, Mitchell, Ruddy (Furness 60), Walker (Rowe 64). Subs: Bobwoski (gk); Matthews.
Boro: Coddington; Johnson, Burn (Capt.), Elsdon, McGinley, Wheatley, Pattison (Kitching 78), Cooke (Jones 56), Jakupovic (McAloon 56), Jowers, Morelli. Subs: Fryer (gk); Maloney.
Referee Alf Greenwood