Hamblin was born in Walworth, London, to a poor family. Despite his lack of money and education, he eventually qualified as an Optician and founded the highly successful firm, "Theodore Hamblin" (which eventually became part of Dollond and Aitchison). He was also a mystic who, at times, could reach a state of consciousness in which, "all care, anxiety and fear vanished, and I felt that I was cradled in Divine Love.... The deep peace of the Eternal flowed through me like a river; yet at the same time it was as though I was being carried along on a stream of Divine Bliss...".

Though successful and prosperous materially, Hamblin also suffered from depression and night terrors. But it was the sudden death of his ten-year old son which precipitated a personal crisis leading him to sever his connections with the business world and seek answers from the "divine presence" he had experienced earlier in life.

Inspired by this presence, he started to write in the early 1920s - his first book, "Within You Is The Power" eventually sold over 100,000 copies. He went on to write a number of other books (see below) and also founded a magazine, "The Science of Thought Review", in 1921, (now called "New Vision") which attracted high-profile contributors such as Joel Goldsmith.

The essence of Hamblin's mystical experience and philosophy was of the omnipresence, omnipotence and all-goodness of God ("The Kingdom or realm of God is with us now and always"). He believed that "abounding health, sufficiency of supply, achievement, accomplishment and joy indescribable are the normal state for man.", and that, to achieve this state, man needed to come into "harmony with Cosmic Law". Over time the emphasis of Hamblin's written work changed from showing people how to change their lives through right thought and faith, to teaching them how to find a living consciousness of God for himself alone.

Hamblin's work is continued to this day through the Hamblin Trust, a registered charity which publishes his books and the magazine, "New Vision" (founded in 1921). The trust, set in three acres of gardens in Bosham, West Sussex, is also a venue for organisations and events promoting healthy living and personal development.