[12] In 2001, he became special assistant to the opposition leader, Sheikh Hasina, and later took on the role of secretary for environment and forest affairs in the Awami League.

[citation needed] During Mahmud's tenure as Environment minister from 2009 to 2013, multiple allegations of corruption surfaced involving the allocation of climate change funds.

The Bangladesh chapter of Transparency International provided with credible allegations of political influence and nepotism in distributing contracts in climate change related projects, and corruption in selecting NGOs to operate the ground works.

[17] On 6 August 2024, Mahmud attempted to flee to Delhi, India after Sheikh Hasina's resignation as prime minister the previous day during the Non-cooperation movement (2024) but was intercepted and detained at Hazrat Shahjalal International Airport in Dhaka.

[18] On 11 August, Bangladesh Financial Intelligence Unit (BFIU) ordered banks to freeze all accounts of Mahmud, his wife, Nurun, and their daughter, Nafisa.

[21] Mahmud is accused of involvement in a murder case related to the death of Mohammad Alam, who was shot during a procession celebrating former Prime Minister Sheikh Hasina's resignation on August 5.

The case statement alleges that the accused, under the direction of Nowfel and Hasan Mahmud, attacked the procession and opened fire, resulting in Alam being shot in the head.

[6] In addition to crimes against humanity during the Student–People's uprising, Mahmud is currently involved in three separate legal cases filed in Chittagong, which include a total of 83 individuals.

[8] The second case, initiated by Md Osman, names Dr. Hasan Mahmud and 31 others as accused of extortion, with 12 unnamed individuals also included.