From 1962 to 1972, Mahtab was a research assistant at the Bangladesh Medical University.[1] She was an assistant professor of medicine at the Bangabandhu Sheikh Mujib Medical University from 1973 to 1976.[1] From October 1976, she was a Senior Consultant in Medicine at BIRDEM.[1] In 1977, she was the Project Manager of the diabetic programme of the World Health Organization.[1] From 1979 to 1980, she was the head of the Bangladesh Institute of Research and Training on Applied Nutrition.[14] In 1982, she was appointed director of the Collaborating Centre for the Prevention and Control of Diabetes of the World Health Organization.[1] From 1983 to 1986, she was member of World Health Organization's Advisory Council for Health and Medical Research of the South-East Region.[1]
Mahtab was the associate professor of medicine at the National Institute of Diseases of the Chest and Hospital from 1980 to 1990.[1] She was a consultant of the Ministry of Health Oman from 1989 to 1991.[1] She was a professor of medicine at the Mymensingh Medical College when she retired on 21 September 1998.[1] From November 2000 to August 2003, chairperson of the International Diabetes Federation's South-East Asia Regional Council.[1] She served as the director of clinical services, research and academy at BIRDEM till September 2003.[1] From September 2003 to December 2004, she was an advisor of the National Healthcare Network project of the Diabetic Association of Bangladesh.[1]
In January 2005, Mahtab was elected member of the National Council of the Diabetic Association of Bangladesh.[1] She was the chairperson of the National Healthcare Network.[1] She was a management board member of BIRDEM.[1] In January 2006, she was appointed a board member of Ibrahim Medical College.[1] She founded the Under 30 Clinic of BIRDEM.[1] She was editor of the Diabetes and Endocrine Journal which is published by the Endocrine Society of Bangladesh and Diabetic Association of Bangladesh.[1] In 2018, she was awarded the Mohammad Ibrahim Memorial Gold Medal.[15]