Coming into the match with two hundreds under his belt, Dhoni carved out a superb unbeaten 85 from 57 balls in energy-sapping conditions to enable his side bag their first CAB-organised title.
Dhoni, who had to retire for some time, was running a fever and badly dehydrated but resumed his innings at the fall of the fifth wicket, to provide the final touches to their victory with five balls to spare.
The India One-day gloveman was well supported by his opening partner Wriddhiman Saha, who made a steady 57.
Earlier, making first use of the wicket after winning the toss, Sporting Union were reduced to 141 for six before old warhorse and former India spinner Utpal Chatterjee solidified the innings, stitching together 107 runs for the seventh wicket with Rahul Deb to help his side post a healthy 263 for seven off 50 overs.
Utpal made an unbeaten 72 from 64 deliveries, punctuated with six hits to the fence.
Charanjit Singh notched 49 while Rahul Dev chipped in with 44.
Anirban Chatterjee was the most successful bowler for Shambazar Club claiming three wickets for 51.
Brief Scores: Sporting Union 263/7 (50 overs). Shambazar Club 267/9 (49.1 overs).