Cowan, one of only two New Zealanders alongside Tana Umaga to have registered a century of appearances for his province and Super Rugby franchise, as well as 50 tests for the All Blacks, is leaving the country this week to link with English side Gloucester.
It was by no means a pretty performance, but it was a valuable win for the Stags that was laid on via the boot of first-five Scott Eade and a lone first-half try to hard-working lock Josh Bekhuis.
Both sides came into the game in contention for a spot in the semifinals in the Championship, but the contest shaped as pivotal for their hopes of making the top four by the end of the regular season.
Northland made it tough on themselves during the opening 40 minutes as they committed a whopping 10 handling errors and conceded six penalties, while the home side only gave up one.
With his side down 13-0, Northland coach Adriaan Ferris rung the changes at half-time as he looked to inject some life in to his troops as the weather progressively got worse in the deep south.
First-five Ash Moeke sparked the Taniwha as he scored his team's first try and set up their second with a well-placed crossfield kick that resulted in replacement hooker Tone Kopelani finding the chalk.
Moeke wasn't able to add the extras to Kopelani's five-pointer and they remained behind 13-12 on the scoreboard. Although they continued to threaten, Eade was able to add another three points to Southland's tally just after the 60-minute mark, which gave them a vital four-point buffer.
Rene Ranger, who collected three test caps for the All Blacks in 2010, looked dangerous with ball in hand and was tough to contain for the Southland defence but was never able to break free for a match-winning try.
Southland's win took them to 15 Championship points and they remain in the hunt for a semifinal spot as they sit in fifth place, while Northland moved to third after they claimed a bonus point for a close loss.
Southland 16 (Josh Bekhuis try; Scott Eade 3 pen, con) Northland 12 (Ash Moeke, Tone Kopelani tries; Moeke con) HT: 13-0
