Little is known about Sinclair's life. Much has been written through conjecture, however, about his possible career as an explorer. In particular, he has been identified by some historians as possibly being the prince Zichmni described in letters by the brothers Nicolo and Antonio Zeno of Venice. According to the letters, Antonio served as the admiral of Zichmni's fleet during a expedition throughout the North Atlantic in the year 1398.
The letters, along with a map of the voyage alledgedly made by Zeno, were published in 1558 in a book by a descendant of Nicolo.
Although the book and map were widely read in Europe at the time of their publication, the exact course of the voyage, as well as whether or not it even occurred, has never been firmly established. Many historians regard it as a hoax, either by the Zeno brothers or by the descendant who later published the book. Moreover, the identification of Zichmni as Henry Sinclair is very controversial.
The most controversial theories speculate that Henry (Zichmni) traveled not only to Greenland but to present-day Nova Scotia, where he may have founded a settlement among the Micmac Indians, and perhaps as far south as present-day Massachussetts and Rhode Island. According to these theories, he may have been responsible for the building of the Newport Tower.