The Sea Dogs became part of the Red Sox system for the 2003 season; previously they were affiliated with the Florida Marlins. The change in affiliation brought success in the 2005 and 2006 seasons as the Sea Dogs went to the Eastern League championship series both years. They won their first-ever title on September 17, 2006, defeating the Akron Aeros 8-5 in a rematch of the series from the previous year. It was the first double-A championship for a Red Sox farm team since 1983 when they were based in New Britain, Connecticut.
The Sea Dogs' home stadium is Hadlock Field, named after long-time Portland High School baseball coach Edison Hadlock. It currently has a seating capacity of 7,368. Hadlock Field is often visited by vacationing celebrities, such as former NFL coach Bill Parcells, former U.S. President George H. W. Bush, and his wife Barbara. In left field stands the Maine Monster, a 37-foot-tall replica of Fenway Park's Green Monster, complete with Coke bottle and Citgo sign. Along the right-field foul line just beyond first base, a picnic area with picnic tables is available for group outings. In 2006, a new right-field pavilion opened above the right-field wall over the Sea Dogs bullpen, offering fans a place from which to catch a home run. Currently all games are carried on a network of radio stations, with the flagship WVAE/WBAE and select TV games on NESN.
Minor league baseball officially returned to Maine on October 4, 1992, when Portland was awarded one of two Eastern League expansion franchises (the other being the New Haven Ravens) to begin play in April 1994. The Sea Dogs signed an affiliation agreement with the Florida Marlins, beginning what would become a nine-season relationship. The city renovated Hadlock Field, transforming what was once a high-school stadium into a modern ballpark. The team won its first game, defeating the Reading Phillies on the road 2-1, with the help of a 14th-inning home run by future major league catcher Charles Johnson. The team opened Hadlock Field on April 18, 1994, losing 7-6 to the Albany-Colonie Yankees. It was the late Robert Ganley, Portland city manager at the time, whose vision led to the construction of the ballpark in a previously run-down section of Portland.
Cartoonist Guy Gilchrist designed the team's logo as well as logos for the Connecticut Defenders, Binghamton Mets, and New Britain Rock Cats. Gilchrist's comic strip Mudpie had a series of strips in which the young cat's family visit the Portland area and attend a Sea Dog game.
- Josh Beckett (2001)
- Clay Buchholz (2006-2007)
- A.J. Burnett (1999)
- Luis Castillo (1996)
- Ryan Dempster (1998)
- Jacoby Ellsbury (2006-07)
- Casey Fossum (2003)
- Adrian Gonzalez (2002)
- Alex González (1996-97)
- Chris Hammond (1994)
- Craig Hansen (2005, 06)
- Liván Hernández (1996)
- Charles Johnson (1994-95)
- Gabe Kapler (2006, on a rehab stint)
- Mark Kotsay (1997)
- Jon Lester (2005)
- Matt Mantei (1995, 1997)
- Cla Meredith (2005)
- Kevin Millar (1996-97, 2002)
- Jonathan Papelbon (2005)
- Dustin Pedroia (2005)
- Brad Penny (1999)
- Hanley Ramírez (2004-05)
- Mike Redmond (1995, 1996, 1998)
- Edgar Rentería (1995)
- Nate Robertson (2002)
- Anibal Sanchez (2005)
- Jared Sandberg (2005)
- Gary Sheffield (1994)
- Kelly Shoppach (2003)
- Wilson Valdez (2002)
- Randy Winn (1997)
- Kevin Youkilis (2003)
(Place is finish in Northern Division)
- 1994: 60-81 (4th), manager Carlos Tosca
- 1995: 86-56 (1st), manager Carlos Tosca
- 1996: 83-58 (1st), manager Carlos Tosca
- 1997: 79-63 (1st), manager Fredi Gonzalez
- 1998: 66-75 (3rd), manager Lynn Jones
- 1999: 65-77 (3rd), manager Frank Cacciatore
- 2000: 71-70 (4th), manager Rick Renteria
- 2001: 77-65 (3rd), manager Rick Renteria
- 2002: 63-77 (5th), manager Eric Fox
- 2003: 72-70 (3rd), manager Ron Johnson
- 2004: 69-73 (4th), manager Ron Johnson
- 2005: 76-66 (1st), manager Todd Claus
- 2006: 72-67 (2nd), manager Todd Claus
- 2007: 71-72 (2nd), manager Arnie Beyeler
- 1995 season: Lost to New Haven, 3-2 in semifinals.
- 1996 season: Defeated Binghamton 3-2 in semifinals; lost to Harrisburg 3-2 in championship.
- 1997 season: Defeated Norwich 3-2 in semifinals; lost to Harrisburg 3-1 in championship.
- 2005 season: Defeated Trenton 3-2 in semifinals; lost to Akron 3-1 in championship.
- 2006 season: Defeated Trenton 3-1 in semifinals; defeated Akron 3-2 to win championship.
- 2007 season: Lost to Trenton 3-1 in semifinals.