If your ancestor served in the British Army or the Honorable East India Company Army, (consisting of separate armies in each of the three Presidencies) then you may find his military records very useful in providing details of his life and history. More details are given on the pages just mentioned and in the article Armies in India. Until 1859, the two armies were entirely separate and their records were stored separately. The East India Company was nationalised after the Indian Mutiny The European regiments of the Presidencies were transferred to the British Army but the Indian regiments with their British officers continued until they became part of the Indian Army in 1895.
See British Army for links to regiments that served in India and articles about them.
The FIBIS search page has an alphabetical list of British Army pensioners who elected to reside in India between 1800 and 1857.
If your ancestor was in the British Army, and you only know that he was in a certain location at a certain time (e.g. he had a daughter baptised in Meerut in 1859), then you may find John Kitzmiller's book In Search of the Forlorn Hope : A Comprehensive Guide to Locating British Regiments and their Records useful. It lists all British Army units by location. This website lists the India related material from the book.
Once you have determined in which regiment he served and at what times, you can learn a great deal about the experiences of your British Army ancestor by obtaining the regimental history of his unit. The National Army Museum may be of help in this respect.