Following the final round of group matches on Wednesday, it was confirmed the County finished fourth in Group Three, following their hard-fought draw at Glamorgan.
The Welsh county finished third in the group, meaning both teams go on to compete in division two of the play-offs, with top two Lancashire and Yorkshire qualifying for division one and the opportunity to win the County Championship title as well as the Bob Willis Trophy.
For Northants the play-offs will simply be about final league position as the six teams in division two compete for seventh to 12th place in the final 2021 Championship table.
The teams in division three will be playing to see who finishes 13th to 18th.
Joining the County and Glamorgan in division two are Durham and Essex from Group One, and Gloucestershire and Surrey from Group Two.
Essex were the county champions in 2019, and also won the Bob Willis Trophy in the truncated 2020 season.
This year's format means that Northants will now play once against the four teams that have qualified from Groups One and Two, but they won't play Glamorgan again, having already played them twice in the group stage.
The teams also all carry a points total into the play-off section, based on their results in the matches against the teams that qualified with them from their group.
So Northants and Glamorgan carry through half the number of points they accrued in the fixtures against each other in the group stage.
So, in Northants' case, having beat Glamorgan at home and drawn away, they carry over 16 points, having claimed 32 points overall, while the Welsh side carry over 11.5 points thanks to their 23-point haul.
That means that before a ball is bowled, Northants will start the division two play-offs in second place in the table, behind Essex who carry over 19 points courtesy of two group stage wins over Durham.
Then comes Northants (16), Surrey (13), Gloucestershire (12), Glamorgan (11.5) and Durham (4).
The fixtures for the play-offs, and who Northants will host in their two home games, have yet to be announced.
The start dates for the four matches are as follows: Monday, August 30; Sunday, September 5; Sunday, September 12 and Tuesday, September 21.