McLennan was born on the Isle of Skye in Scotland. In 1801, he and a large group of family and friends immigrated to Richmond County, North Carolina, later moving further south to Florida. The McLennans were Gaelic speakers and Presbyterians. Around 1820, they were invited by Yuchi Chief Sam Story to become the first white settlers in Walton County, Florida. In December, 1834, Neil and his brothers Laughlin and John, along with some of their families and friends, immigrated to Texas, sailing a self-built three-masted schooner from Pensacola, Florida, to the mouth of the Brazos River, where they arrived in March, 1835.
After a short stay at present-day Columbus, Texas, Neil obtained a grant of a league of land at McLennan's Bluff (a mile west of present-day Rosebud, Falls County) in Robertson's Colony in July, 1835, where he and his group settled. Laughlin McLennan and his wife and mother were killed in an Indian raid the following winter, and John was likewise killed in 1838, after which the McLennan group moved to the Robertson Colony's headquarters of Nashville-on-the-Brazos in present-day Milam County.