Michael Cass, National Labor Relations Board examiner who supervised the balloting, said Friday's vote was 145 to 90.
Workers who supported the union have said they wanted more of a say in how the nonprofit tourist attraction is run. Management had fought the union drive.
Mystic Seaport President Steve White said the organization is pleased with the vote. He said the next step is to "begin the healing process" and address workers' concerns.
In the recent recession, Mystic Seaport cut its budget by $3 million, partly through a 10% staff cut in two rounds of layoffs in 2008 and 2009. It also closed for six weeks in the winter.
Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.