North Otago had plenty of possession in both halves but a lack of variation often saw attacking movements shut down.
The defence from both sides was powerful, and the match was marred to some extent by a multitude of replacements.
North Otago first five-eighth Ben Palmer impressed, in his first start, with his ability to take the ball to the line. His distribution was accurate and his goal kicking produced 11 invaluable points.
North Otago lost centre Lemi Masoe with a groin injury. He was replaced by Joe Marston, who went to the left wing, and Tenina Sauileoge joining brother Ekeroma in midfield.
The Old Golds pack played with renewed vigour, providing ample ball for the backline, as Don Lolo, Manu Taiti and Sale Pi'i dominated the lineouts.
Thames Valley was well served by halfback Richard Judd, while young winger Josh Turner showed his versatility when he shifted in to first five to replace veteran David Harrison. Turner made a brilliant break that opened up the North Otago defence for a try, only to be called back after he was ruled to have stepped into touch.
Locks Brett Ranga and Kent McFarlane were avid toilers and hooker Blake Hill was lively.
North Otago coach Mike Mullins was a relieved man at fulltime.
''It was our second win of the year so, hopefully, we'll keep building on this,'' Mullins said.
''We showed some lapses on defence, allowing Thames Valley back into the game, and that's what this game was about in the end. We'd have them under pressure and then just give away a soft penalty or missed tackles.
''But the hard work has paid off. The first 20 minutes of the game, we dominated, and we did the simple things really well.
''Our work as a pack was a lot better but those defensive lapse almost cost us again.''
North Otago took the lead in the sixth minute when a Robbie Smith kick was chased by winger Simeli Koniferedi, who forced a turnover. From the melee, hooker Ralph Darling was awarded a try, which Palmer converted from wide out.
Palmer (two) and Harrison kicked penalties as the first half ended with North Otago holding a deserved 13-3 lead.
Palmer added his third penalty nine minutes into the second half to make it 16-3 before Thames Valley began to make inroads into North Otago's lead.
In the 57th minute, a defensive lapse allowed Thames Valley winger Jason Laurich to break two tackles and race 40m for an unconverted try.
North Otago held on until the 75th minute, when Thames Valley swept 60m down the field. Hooker Hill burst over the line from from the breakdown, and Turner's conversion brought his side within a point of North Otago.
A minute from fulltime, Koniferedi took North Otago back inside the opposition 22m, and Stephen Fenemor, who had replaced Palmer, kicked a penalty to seal the win.
Buller maintained its unbeaten run when it beat defending Meads Cup champion Mid Canterbury 29-19 in Ashburton.
Buller is now three points ahead of South Canterbury, which beat Poverty Bay 32-28. Horowhenua-Kapiti retained third place with a 12-9 win over Wairarapa-Bush.
In the middle four (Lochore Cup) group, Thames Valley is top with 15 points, ahead of King Country, which beat West Coast 15-10, Mid Canterbury and North Otago. Wanganui (9), Wairarapa-Bush (3), West Coast (2) and East Coast (0) make up the bottom four.
By Terry O'Neill.
Heartland Champs
The scores
North Otago 19
Ralph Darling try; Ben Palmer con, 3 pen, Stephen Fenemor pen
Thames Valley 15
Jason Laurich, Blake Hill tries; David Harrison pen, Josh Turner con
Halftime: North Otago 13-3.
