Simba SC needed a 90th-minute strike by Luis Miquissone to defeat Azam FC by a solitary goal at Majimaji Stadium in Songea to qualify for the FA Cup final.
It was a win that ensured Wekundu wa Msimbazi play rivals Yanga SC in a Kariakoo Derby final.
Saturday's opponents had faced off last season at the quarter-finals stage with Mnyama winning the tie 2-0 courtesy of goals from Clatous Chama and John Bocco, en route to meeting Yanga in the semis, where they also won 2-0 to reach the final.
Despite the defending champions starting the match better than their rivals, they struggled to get clear-cut chances. The best chance of the first half, arguably, was in the 30th minute.
Azam won a corner which was brought in by Selemani, Salum Abubakar controlled it before unleashing a powerful shot which Tanzania international Aishi Manula managed to parry.
With Mnyama almost assured of successfully retaining the league title, it was the Ice-cream Makers who pushed harder for the win but the former were calm on the ball.
When it seemed the match will go to penalties, the Mozambique attacker was fed the ball in a good area and he took no time to release a powerful shot that the Uganda keeper Mathias Kigonya could not stop.
It was the goal that sealed Simba's place in the final where they will play Yanga.
Timu ya Wananchi secured their place in the final after a hard-earned 1-0 win against Biashara United on Friday.
Yanga went into the fixture seeking their first final in three years and they were rewarded in the 21st minute when striker Yacouba Songne scored the all-important goal to sink Biashara.
The overall winner will be representing Tanzania in the Caf Confederation Cup alongside the team that will finish third in the Tanzania Mainland League.
The East Africa nation will enter four teams in Caf competitions next season owing to good performance by Simba, who reached the quarter-finals of the Caf Champions League, where they fell 4-3 on aggregate to Kaizer Chiefs of South Africa.
Simba XI: Aishi Manula, Shomari Kapombe, Mohamed Hussein, Joash Onyango, Pascal Wawa, Taddeo Lwanga, Clatous Chama, Yassin Mzamiru, John Bocco, Rally Bwalya
Azam FC XI: Mathias Kigonya, Nicholas Wadada, Bruce Kangwa, Aggrey Morris, Daniel Amoah, Bryson Raphael, Ayoub Lyanga, Mudathir Yahya, Obrey Chirwa, Salum Abubakar. Iddi Selemani