In the first phase of his legal career, he worked as a junior associate with prominent Bangladeshi lawyer Syed Ishtiaque Ahmed.
[7] In December 2005, Mahmud criticized the government passing an amendment to the Bangladesh Telecommunications Act, 2001 allowing law enforcement officers to eavesdrop phone calls and said that it created the potential for abuse.
[8] He criticized the Bangladesh Nationalist Party appointing 17 additional judges to the High Court Division with the input of the chief justice and accused the government of politicizing the judiciary in August 2006.
[10] In November 2006, he rushed to the police station with Barrister Amir-Ul Islam following the arrest of activists of Peshajibi Nari Samaj (Professional Women's Society) who were marching to the Election Commission with a memorandum.
[14] In 2020, Mahmud represented Matiur Rahman, editor of Prothom Alo, in a case filed over the accidental death of college student at a program of the newspaper.