Turnbull slotted his first goal of the season midway through the first half before producing one of his trademark long-range efforts after the break.
James Forrest wrapped up a 7-2 aggregate victory which sent Celtic through to a play-off against AZ Alkmaar.
Three stands at Celtic Park were full – the main stand capacity is still limited by Covid protocols – and the sell-out crowd watched Ange Postecoglou's side secure a comfortable third consecutive win.
They roared their approval as Celtic dominated much of the opening half, although there were some second-half scares.
Celtic could have been ahead inside 30 seconds. Tom Rogic split the Jablonec defence and Kyogo Furuhashi only had the goalkeeper to beat but was dispossessed as he tried to go round him.
The Japan forward continued to play on the last shoulder and keeper Jan Hanus had to be quick off his line to stop him getting on the end of through balls from Turnbull and Rogic.
Forrest also set up the striker to net but the offside flag halted the celebrations.
Turnbull had a powerful strike held at the second attempt and the pressure paid off in the 25th minute.
A move broke down when Ryan Christie's return pass to Turnbull was cut out, but Greg Taylor charged forward to win it back and slipped in the former Motherwell man, who slotted home with the outside of his boot.
Jablonec were getting men forward and pressing Celtic at the back but Joe Hart only had one comfortable save to make in the first half.
Celtic had a let-off in the opening moments of the second half when Carl Starfelt went in with a reckless penalty-box challenge which appeared to hurt his opponent. The referee saw nothing wrong with it and the hosts survived when Jakub Povazanec shot just over.
The home side regained their spark and Callum McGregor burst forward from the base of midfield to join an attack and fed Turnbull 25 yards out. The Scotland international took a touch and powered a shot into the corner of the net.
The fans saw another example of Postecoglou's style of football when Celtic worked the ball patiently around the visitors' press deep inside their half. They soon broke forward and Furuhashi was on the end of another through ball only for the flag to halt the move.
The supporters also hailed two excellent stops in quick succession from Hart, the first after a poor back pass from McGregor and the next from the resulting corner. After the fans watched their team struggle in the goalkeeper position for the majority of last season, the second save in particular was cheered like a goal.
They soon had another of those to celebrate. Christie cut inside and played in substitute Odsonne Edouard, whose effort was well saved. Forrest followed up and slid in to stab home the rebound.
With the game well and truly won, Postecoglou made a number of substitutions.
The game was briefly held up by a one-man pitch invasion that will likely earn Celtic another UEFA fine.
Celtic soon had another let-off when Hart lost possession inside his box, but the home defenders converged on the Jablonec player to snuff out the danger.