It was a tough encounter for both sides due to the monsoon rains which lashed Kolkata and made the ground and underfoot conditions heavy.
Mohun Bagan expectedly had more of the game with young Azharuddin Mallick having the best chance for the Mariners, but it stayed goalless at the break.
Mohun Bagan upped the pressure after the break but they had to wait until the 79th minute for the winner to be scored by substitute striker Ajay Singh.
With the win Mohun Bagan moved to the top of the standings with nine points from three matches, but only due to a superior goal difference than their archrivals East Bengal.
MATCH STATS
Mohun Bagan AC 1-0 Southern Samity – Goals: 1-0 Ajay Singh (79′)
Mohun Bagan: Arnab Das Sharma – Sarthak Golui, Sanjay Balmuchu, Raju Eknath Gaikwad, Tanmoy Ghosh – Prabir Das, Thangjam Saran Singh, Rayner Fernandes (57′ Robinson Singh), Tapan Maity (68′ Ajay Singh) – Azharuddin Mallick, Darryl Duffy (90+’ Bikramjit Singh)
Southern Samity: Sourav Chakraborty – Sumon Hazra, Deepak Kumar Mondal, Echezona Celestine, Safar Sardar – Antoni Saren, Gautam Kujur, Basanta Singh (55′ Nagen Tamang), Snehasish Chakraborty (83′ Snehasish Dutta) – Ashim Biswas, Taurus T. Manneh (79′ Rooney Eva)
Referee: Pranjal Banerjee
Man of the Match: Azharuddin Mallick (Mohun Bagan)