Although a local obituary called the 24th Sultan of Johor ''one of the worst of Malaysia's sultans, a violent, often brutal and impulsive man'', others spoke of his contributions to the nation and his people's welfare. Many of his subjects spoke well of him and said he was easy to approach for help. A friend defended his reputation, saying that the sultan did not like people to worship him - they only had to bow as a mark of respect when greeting him.

Mahmud Iskandar Almarhum Sultan Ismail was born on April 8, 1932, in Johor Bahru, Malaya. He was the first surviving son of the 23rd Sultan of Johor, Ismail ibni Sultan Ibrahim, and his first wife, Ungku Tun Aminah binti Ungku Paduka Bena Sri Maharaja Utama Ahmad, and was known as Tunku (or Lord) Mahmud Iskandar.

Tunku Mahmud went to a local primary school and the English College in Johor Bahru. In 1952, he came to Sydney to attend Trinity Grammar School. After graduating in 1953, he went on to the Isle of Wight for three more years of schooling.

He returned to Malaya in 1956, joined the Johor civil service and served in the departments of district affairs, land and treasury.

In 1959, he was named Tengku Mahkota (or crown prince) of Johor and married an Englishwoman, Josephine Trevorrow. They had four children, including Ibrahim Ismail, who succeeds his father as sultan. The marriage ended in divorce in 1962 (and is often not mentioned in official histories). Meantime, the prince had married, in 1961, Zanariah binti Almarhum Tunku Ahmad of the Kelantan royal family and they had six children.