Graham Westley's side had not previously played in the Football League prior to this season, but can now celebrate back-to-back promotions.
Mousinho's powerful 20-yarder proved enough to settle a scrappy affair played in blustery conditions, despite an improved second-half showing from Torquay.
The Gulls barely threatened in the first half and could have few complaints at heading into the interval a goal down.
However, Stevenage were largely on the back foot thereafter and Torquay came close to grabbing an equaliser with 15 minutes remaining when Jake Robinson's shot grazed the crossbar.
Mousinho returned from suspension to take his place in Stevenage's midfield, with Darren Murphy dropping to the bench as a result. Centre-half Jon Ashton, who had been a major doubt for the game due to a calf injury, was named among the substitutes.
Torquay manager Paul Buckle named the same XI that started both legs of their semi-final victory over Shrewsbury.
After kick-off had been delayed by 15 minutes due to traffic congestion, it was Stevenage who enjoyed the better of the opening period.
Craig Reid saw a goalbound effort blocked by Guy Branston in the fourth minute after good work down the left from Darius Charles.
The hard-working Charles then got the better of Branston on the opposite flank and cut the ball back to Mousinho, although his right-footed shot failed to seriously trouble Torquay goalkeeper Scott Bevan.
Winger Robinson represented the Gulls' biggest threat early on and saw two crosses cleared from promising positions.
However, it was Stevenage who continued to look the most likely to make a breakthrough and Charles fired narrowly over in the 27th minute after Bevan had flapped at a cross from Lawrie Wilson.
Billy Kee then went close for Torquay with an ambitious long-range effort, before Charles threatened again as his header from Stacy Long's cross bounced into the ground and over the bar.
With four minutes of the first half remaining, Stevenage claimed a deserved lead as Mousinho surged towards the edge of the box before drilling a powerful shot across Bevan and into the bottom-left corner.
However, Torquay came close to an immediate leveller when Branston headed wide from a corner and they went on to pose much more of a threat in the second half.
Kee forced Chris Day into a sharp save at his near post and Chris Zebroski fired into the side netting after a rare mistake from Stevenage skipper Mark Roberts.
Borough substitute Darren Murphy then missed a wonderful chance to double his side's lead as he failed to connect cleanly with a free header from Long's left-wing corner, before the lively Robinson struck the bar for Torquay with Day beaten.
The Gulls threw men forward in desperation as the final whistle approached and Branston should have done better when volleying high and wide after his initial effort had been blocked.
Stevenage held on with relative comfort in the closing moments, however, and can now look forward to visiting the likes of Hillsborough, Deepdale and Bramall Lane next season.
Stevenage 1-0 Torquay: Match Report - view commentary, squad, and statictics of the game as it happened.