The 34-year-old brought the milestone up in a low-key way in the Plunket Shield match against Auckland when he pushed a delivery from Tarun Nethula gently down the ground for a single.
There was some polite applause from a sparse crowd at the University Oval, and the Auckland team graciously put their hands together.
It was Cumming's 15th first-class hundred for Otago and his 17th overall.
He has passed Ken Rutherford's haul of 14 centuries for Otago and is just two away from equalling Bert Sutcliffe's total of 17, an achievement the right-hander would celebrate with gusto.
"I'm not big on statistics but that is something I've kept an eye for the last year or so," he said.
"Runs come about if you play long enough but you have to bat well to score hundreds, and it is nice to get to No 2.
"I don't know what it would be like to draw level with the great Bert Sutcliffe but I would like to know."
Otago reached 274 for five at stumps on day one, with honours in the match about even.
Neither side really imposed itself on the match. Auckland's fielding was average at times and the seamers bowled well in spurts but failed to assert themselves on the opposition for prolonged periods. There was always a wide or half-volley to help release the pressure.
Otago, for its part, was content to defend first and attack third.
Cumming provided some relief when he rushed through the 80s with two crisp sixes, and there was a healthy splattering of drives to excite the senses throughout his innings.
Soon after he reached the milestone, though, he got an edge to slip and departed with 102 runs to his name. He has scored 4806 runs for Otago and is the fourth-highest run-scorer behind Sutcliffe (6038), Bruce Blair (5057) and Rutherford (5051).
Leighton Morgan's long vigil came to an end when he was trapped in front by Gareth Shaw in the next over. He combined with Cumming in a 92-run stand for the second wicket and scored 29 off 136, an innings diplomatically described as patient.
Opener Aaron Redmond had an injury scare when he was on 12 and he left the field in discomfort.
Redmond has spent most of the summer on the sidelines nursing a muscle strain and there was concern he had aggravated it, but those fears were allayed when he returned to the crease at the fall of Cumming's wicket.
He failed to add to his score, though, steering a delivery from Shaw into the slips after he attempted to leave the ball but left the periscope up.
Otago's innings was starting to unravel as it slumped from 171 for one to 171 for four.
Batting all-rounder Sam Wells defied the usual strategy of shutting up shop with stumps in sight and whacked Lance Shaw for consecutive boundaries, the first through cover and the second a lovely off drive.
His undefeated partnership of 65 with Derek de Boorder helped give the home side some momentum.
Shaw and Nethula picked up two wickets apiece but Nethula, a legspinner, blotted his copybook with 10 no balls. A sin for a slow bowler.
This morning's session will be crucial for both teams.
