montagu was the eldest son of james bertie, 1st earl of abingdon and eleanora lee. though young not yet matriculated, he was chosen captain of the company of militia foot raised from christ church during the monmouth rebellion. through the influence of his father, he was made a freeman and common councilman of woodstock in 1686, and a freeman of oxford in 1687. on 22 september 1687, he married anne (d. 28 april 1715), the daughter and coheiress of peter venables (d. 1679), baron of kinderton. he shortly afterwards assumed the additional surname of venables. at the january 1689 election, he was returned, though underage, as a knight of the shire for berkshire on his father's interest. during the year, he was made a bailiff of oxford and appointed a deputy lieutenant of oxfordshire, holding that office until 1701.