Ambrose Gracey was a sea captain who built Gracey Manor with the wealth he made from merchant-work in New Orleans (assisted by the smuggler/pirate Jean Lafitte). He married the French aristocrat Juliet Desmoulins and fathered George Gracey.
Ambrose died in a carriage accident and left his shipping empire to George to run with George going on the be the father of Edward Gracey.
Behind the scenes
Ambrose Gracey's being a sea captain is inspired by the old backstory involving Bartholomew Gore that the Imagineers had created as a possible backstory for the Mansion and the Ghost Host before dismissing it, which nonetheless survived as the Mariner character (and his Culpepper Clyne expansion), as well as inspiring the backstory of the SLG comics' Master Gracey and that of the Ghost Host in Nuptial Doom: The Bride in the Attic.