[1][4][5][6] Chalmers qualified in medicine in the mid-1960s, and then practised as a clinician in the United Kingdom and two years (1969–1970) in the Gaza Strip.

[7] In the mid-1970s, he became a full-time health services researcher with a particular interest in assessing the effects of care.

[8] There, Chalmers led the development of the electronic Oxford Database of Perinatal Trials (ODPT)[9] and a collection of systematic reviews of randomized trials of care in pregnancy and children published in the two-volume Effective Care in Pregnancy and Childbirth,[10] co-authoring its summary, Guide to Effective Care in Pregnancy and Childbirth.

[9][11] Subsequently, he became founding editor of the James Lind Library, which documents the history and evolution of fair trials of treatments, and helped to establish the James Lind Alliance, a non-profit organization that "aims to identify the most important gaps in knowledge about the effects of treatments".

[12] Chalmers inspired champions all over the world leading to the development of the Cochrane Collaboration and by 2011 this collaboration had nearly 30,000 volunteers contributing towards summarising research evidence to improve health.