Lamar was not uncommon as a first name in the Southern part of the USA during the 19th century. It was originally given in honor of the Lamar family, which produced prominent political figures in both
Georgia and Texas. For a couple of examples see:
http://www.lsjunction.com/people/lamar.htm
http://bioguide.congress.gov/scripts/biodisplay.pl?index=L000030
The American Lamar family were probably of French origin and the surname is an Anglicization of the French surname Lamare, which originally designated residence by a pond.