Young Tom grew up on the family farm in Brewer with his four older siblings: Joshua Lawrence (born in 1828); Horace Beriah (1834); Sarah Brastow (1836); and John Calhoun (1838).

[citation needed] Thomas Chamberlain was soon placed in the newly formed 20th Maine Infantry along with his brother Joshua, who was made Colonel of the regiment.

The brothers fought at the Battle of Fredericksburg, suffering light casualties in the assaults on Marye's Heights, but they were forced to spend a miserable night on the freezing battlefield among the many wounded and dead from other regiments.

Chamberlain's brother Joshua recognized the dire circumstances and ordered his left wing to respond to the rebels by charging downhill with fixed bayonets, thus ending the Confederate attack on the hill.

At the Siege of Petersburg, the 20th Maine was in reserve, while Joshua (against his better judgment) led his Pennsylvania Bucktail brigade in a charge on a section of the Confederate defenses known as Rives's Salient.

As Thomas waited, Dr. Shaw, with Dr. Morris W. Townsend of the 44th New York, worked all night to try to save Joshua Chamberlain's life.

Remarkably, Col. Chamberlain survived to enjoy his "on the spot" promotion to brigadier general, although he never returned to full fitness.

At the end of the war, the 20th Maine marched from Appomattox, Virginia, on May 2, reaching Washington, D.C., on May 12, where it was then finally mustered out of service on July 16, 1865.

[citation needed][2] Chamberlain was a character in Michael Shaara's Pulitzer Prize-winning historical novel, The Killer Angels.