In 2004, Siddique was called to bar by Gray's Inn.[3] he joined the Bangladesh Bar Council in 2005.[3]
In 2012 and 2013, Siddique was working as the defence counsel of Ghulam Azam at the International Crimes Tribunal.[5][6] Azam, former Ameer of the Bangladesh Jamaat-e-Islami, had been charged with war crimes for his role during the Bangladesh Liberation War while Barrister Tureen Afroz was the public prosecutor.[5] He co-defenders were Ehsan Siddique, and Shishir Monir.[6] He worked as the defence lawyer of Delawar Hossain Sayedee in 2014.[4]
Siddique filed a petition challenging the legality of the Section 57 of the Information and Communication Technology Act, 2006 in 2015.[7]
Siddique is a partner at The Law Counsel, which he joined in 2004.[3][8] He is a founding trustee board member of the Centre for Law, Governance, and Policy.[3] Siddique fought a legal case to ensure madrassah students have access to higher education in Bangladesh.[3] He has represented Bangladesh Biman, Chittagong Development Authority, Civil Aviation Authority of Bangladesh, Meghna Group of Industries, National Board of Revenue, and National Curriculum and Textbook Board.[8] He is a member of the Bangladesh Supreme Court Bar Association.[9]
Following the fall of the Sheikh Hasina led Awami League government, Siddique was made a member of the Constitutional Reform Commission.[1][10] The Commission is led by Ali Riaz.[11] The Constitution Reform Commission submitted its report in January 2025, recommending that equity, human dignity, pluralism, and social justice replace nationalism, secularism, and socialism.[12]