The 2015–16 Mohun Bagan A.C. season was the club's 126th season since their formation in 1889 and their 19th season in the I-League which is India's top football league. During this season, Mohun Bagan started the I-League as the defending champions (having won it last season). The team finished 3rd in the Calcutta Football league and runners-up in the I-League. Mohun Bagan were defeated by Shandong Luneng of China in the preliminary round 2 of the AFC Champions League and were eliminated from the competition at this stage. They bowed out of the AFC Cup in the round of 16 where they were defeated by Tampines Rovers of Singapore. They were crowned Champions of the Federation Cup after they defeated Aizawl in the final.

On 9 August, Bagan started their season in the Calcutta Football League. Their start though wasn't as promising as their rivals, East Bengal.
They started their campaign against Mohammedan but weren't strong enough to score against them even after a Mohammedan player was sent off in the first half. The game finished with a scoreline that read 0–0. Their next match was against SAI Darjeeling. This match again was a disappointment for Bagan as they failed to score against a comparatively weaker opponent. This match ended 0–0 even though SAI Darjeeling played half of the game with u-20 players.
They played their next match against Southern Samity in which Bagan registerd their first victory of the season. The Green and Maroon brigade won the match 5–2 in which Dudu Omagbemi scored a hat-trick and Katsumi Yusa scored the other two. They continued their winning ways in the next match against Bengal Nagpur Railway with a 3–0 drubbing of the weaker opponents. Katsumi Yusa scored a brace containing a free-kick goal and the other one was added by Dudu Omagbemi.
On 25 August Bagan tasted defeat for the first time in the season losing 1–0 against Army XI.
They returned to winning ways in the next game against Tollygunge Agragami where they defeated them 3–1 courtesy two goals from Dudu Omagbemi and another from Lalkamal Bhowmick who scored after many games to be the first Indian to score for Bagan this season. On 31 August, Bagan's match against Aryan was abandoned after 20 minutes of play due to heavy rain and the scoreline hung at 0–0.

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