The visitors got to a very healthy position at 263 for three. But Otago rallied, snapping up some late wickets to claw its way back into the game a little by stumps on day 1.
Michael Rae brought plenty of energy and nabbed three wickets in his return from a hamstring injury.
But the day belonged to Johnson, who cut the ball backward of square to bring up his second first-class century in his 10th game.
He was dropped on one and again on 81. But it is the runs that actually count in the end.
The 24-year-old right-hander scored all round the ground but was particularly strong square of the wicket.
For the most part Otago squandered its opportunity to make early inroads into the Wellington batting line-up.
The Volts were not able to hit a consistent length and build pressure that way.
Instead the opening pair of Rachin Ravindra and Devan Vishvaka, who was on debut, rattled on a 113-run opening stand.
Vishvaka leaned on good hand-eye co-ordination rather than solid technique to blast nine fours and a six.
He favoured the hoof to midwicket but hit a couple of drives down the ground when the ball was over-tossed.
Ravindra played in orthodox fashion. He dispatched a pleasing series of drives and swivelled neatly into pull shots when given the opportunity. Anything near his pads went as well.
But regardless of their various methods, both were out for 56 just before the lunch break.
Wellington resumed on 119 for two but lost its way for a period.
Jarrod McKay trapped Jakob Bhula lbw for six and Johnson was dropped early.
Rae found the edge of his bat but it was grassed at second slip.
Otago bowled a lot better during the afternoon session. But Johnson and Tim Robinson, who is also on first-class debut, dug in and got through the session without further drama.
That theme continued for a long stretch in the final session and the frustration mounted for Otago.
Johnson had a second life. Jacob Cumming missed a tough chance off Rae, who was hustling in and trying to muscle a wicket.
To make matters worse, he slashed the next delivery over the top of the slips. It did not improve the bowler’s mood.
But the eventful over had another twist. Johnson got off strike and Robinson launched into a drive but was bowled by a delighted Rae for 54. It was a nice knock first up and the pair had put on 144.
Rae struck again quickly. Tom Blundell squeezed a catch to Dale Phillips, and wrist spinner Michael Rippon levered out another.
The visitors had slumped from 263 for three to 266 for six.
In Palmerston North, Canterbury is 341 for eight at stumps on day 1 of its game against Central Districts. Cole McConchie is 136 not out and Ray Toole has taken four wickets.
No play was possible on the opening day of the top of the table clash between Northern Districts and Auckland in Whangarei.
