Not only did his father's wise teachings push him towards his calling, but also the inspirational poem, "The Day of Judgment". Although he was young he sensed God's heart for people. The reality of a vision from God confirmed his mission in life. His response to a vision revealed to him by God proved similar to Isaiah's, in the Old Testament. As a result of a near death experience Alexander's faith in God was strengthened. Consequently, he felt a heavy burden for the "lost" people.

He graduated from St. Andrews College in Edinburgh with a ministry license. Soon after, he married a wonderfully devoted woman, Anne Scott Drysdale. Once God confirmed his ministry, they sailed for Calcutta, India in 1829. Over a year later, Duff started a school in English for all Hindus and Muslims. However, this school was not quite the same as most schools in India. Biblical views added to each class began Alexander Duff's evangelism to Hindus and Muslims.

Raj Rammohum, a Hindu who detested idolatry, seemed to be open to God. Although he did not accept the Christian religion, he played an immense role in the opening of Duff's school. It took much time and teaching but, slowly, and surely Hindus and Muslims converted to Christianity.

As Duff aged, his health dwindled away. The more and more he worked the less time he was able to spend in Calcutta. Eventually he moved to Scotland to get better medical care. Sadly, he died of old age on February twelfth of 1878.