The right hander, who captained the New Zealand under-19 side on its tour of England this year, easily looked the best of the batsmen on display after arriving at the wicket with Northern in a minor pickle at 62 for two.
He raced to 24 runs off 26 balls, including whacking a lovely six over long on off Nathan McCullum.
But when Michael Parlane got himself out on 41, Williamson dug in to help guide his side to 223 for seven at stumps on day one of Northern's four-day State Championship match with Otago at the University Oval yesterday.
"It depends on the situation of the game as to how you bat and once Michael got out I had to play a little more conservatively and accumulate the runs until the other batsmen got in," he said.
Until yesterday, Williamson had scored just two runs in first-class cricket and was averaging one.
So, to play such a crucial role for his side was a thrill.
"I'm quite pleased but I should have kicked on. It was my job and I didn't really do that."
Williamson finally came unstuck when he hesitated and went back to a delivery from off spinner Nathan McCullum and got a faint edge.
Poor weather delayed the start of play.
But once the match got under way at 12.15pm the home side immediately grabbed the initiative with Neil Wagner striking in the first over.
His victim, the unlucky Cameron Merchant, did his best to defend a delivery which swung away and took the edge.
The disconsolate batsman made the worst possible start to the summer and trundled off in despair after facing just one ball.
Wagner, in stark contrast, revelled in the moment.
The South African-born left-arm swing bowler was making his debut for Otago and arrived in Dunedin with a big reputation, having taken 89 first-class wickets at 18.29.
He did little to harm his reputation with a solid return of two for 34 from 15 overs.
While not express in pace, he is reasonably sharp and had Northern Districts' wicketkeeper Peter McGlashan hopping around to avoid a few well-directed bouncers.
Ian Butler will want to forget his first delivery for Otago - a half volley on middle and leg which BJ Watling effortlessly dispatched to the long on boundary.
The former Black Cap pace bowler redeemed himself shortly after the lunch break when he picked up the key wicket of James Marshall.
The top order batsman looked to carve Butler through point but only managed to chop the delivery on to his stumps.
With Marshall gone, Otago turned its focus to snaffling Parlane's wicket.
He had played quite nicely, getting through to 41 before he took an awful swipe at a wide delivery outside off and feathered a catch to the wicketkeeper, Derek de Boorder.
Teetering at 108 for four, Williamson and McGlashan added 79 for the fifth wicket to help prop up the innings.
McCullum broke the partnership when he trapped McGlashan in front and 11 runs later removed Williamson.
Northern will resume today with Graeme Aldridge on 18 and former Otago player Bradley Scott on five.
Wellington captain Matthew Bell did his best to make up for lost time against State Championship rival Canterbury yesterday.
Bell hit his stride early in scoring 88 not out as Wellington ended a heavily truncated first day of the four-day fixture on 155 for two at the Basin Reserve.
The former international had spent much of the day under cover as the two teams sheltered from the rain, which eventually allowed just 47 of the scheduled 100 overs to be bowled.
Auckland hit the ground running last week and its momentum did not slow yesterday as it had the better of Central Districts in Napier.
The host ended the rain-interrupted opening day of the four-day fixture on 191 for six after Auckland won the toss and inserted CD at Nelson Park.
Play did not start until shortly after noon due to morning rain and the batsmen's progress was always circumspect.
