In 1850 he was elected a member of the U. S. House of Representatives representing the state of Louisiana in its First District, which at the time encompassed part of New Orleans and everything east of the city but south of Lake Pontchartrain.

About this time he served as an election commissioner for the city of New Orleans.

In 1884, when the vote had again been essentially restricted to only white males in Louisiana, St. Martin was returned to the house, representing the First District which had virtually the same boundaries it had had when he was first sent to congress more than 30 years before.

St. Martin was born in St. Charles Parish and died in New Orleans.

The long hiatus between St. Martin's two terms in the U.S. House of Representatives can be partially explained by the intervening Civil War.