Napier was descended from a junior branch of the Scottish Napiers, lairds of Merchistoun. His father made his fortune as a London merchant, acquiring both a baronetcy and the Luton Hoo estate in 1611, while his uncle Richard achieved fame as a physician and astrologer.
In 1626 Napier was elected to represent Corfe Castle in the second Caroline Parliament, probably on the nomination of a local landowner, his kinsman Sir Nathaniel Napper*.
In the following decade, Napier reaffirmed his social credentials by marrying a daughter of the earl of Bridgewater. He represented Peterborough in the Long Parliament, but his membership was suspended for much of the Civil War period due to his wavering allegiance, and was finally terminated by Pride’s Purge.
