Northeast now has the two highest individual scores for Glamorgan after hitting 410 in 2022, with David Lloyd and Steve James also having treble centuries to their credit.
Glamorgan accumulated relentlessly in the Saturday morning session as Northeast passed the county’s previous best individual score against Middlesex, 226 by newly appointed chief executive Dan Cherry.
There were few fireworks in the first 90 minutes as Northeast and Ingram settled for four batting points.
But Northeast stepped up the tempo with three legside sixes off Ryan Higgins before lunch as Middlesex went on the defensive.
Northeast was mostly impeccable but offered a gully catch and a stumping chance late in his innings, before setting his sights on the ground record.
The closing stage of Glamorgan’s highest team score against Middlesex was a one-sided affair as Ingram opened his shoulders to blitz into three figures after a mostly restrained knock by his standards, hitting 12 fours and four maximums.
With 471 needed to avoid the follow-on – and in turn condemn the match to an almost certain draw – Stoneman and Sam Robson showed little sign of scoreboard pressure getting to them.
Glamorgan’s bowlers, featuring newcomer Mir Hamza and loanee Craig Miles, toiled as the openers added 79 before Miles produced a peach of a delivery to bowl Robson through the gate for 43.
Left-handers Stoneman and Max Holden, both short of runs in division one in 2023, had few alarms in the closing stages as bat continued to dominate ball.
Glamorgan captain Sam Northeast told BBC Sport Wales:
“To break a record like that is beyond my wildest dreams. I got pretty nervous at the end when I was nearing it as it’s a privilege to play here.
“I didn’t think about it until I was on about 330, a member told me as I was walking out [after lunch] and I forgot about it, then started thinking about it again when I got near it.
“I was thinking about the game situation but then with the personal milestone, I had to make sure I got it.
“When I feel good, I stay in my bubble and play at my tempo. The pitch is pretty good and the Kookaburra ball doesn’t do a huge amount so it was perfect conditions to get a big score like at Leicestershire [in 2022].
“It was a special day, but we’ve got a match to win and it’s not going to be easy. We’ve got to find ways and be imaginative, though I thought the boys bowled really well.”
Middlesex batter Mark Stoneman said:
“We’re miles behind in this game but there are points to be had. As a batting unit it’s our job after a pretty dreadful year last year to turn things around.
“We’ve got to get a foothold in the game, to try to keep Glamorgan out there as long as possible. We’ve got to get partnerships going and do some hard work.”