Chamberlain grew up in Lincoln, Nebraska. At age 3, his parents divorced, and his father Harlan, a counselor at the Nebraska State Penitentiary, took custody of Joba and his older sister Tasha. The family lived in poverty, with Harlan often pawning his possessions to buy things for the children. Joba served as a ball boy for Lincoln Northeast High School's state championship basketball team, and eventually graduated from Northeast. He did not jump straight to college; to help pay the bills, Joba briefly worked for the city of Lincoln's maintenance department.

Chamberlain's father was born on a reservation for the Winnebago American Indian Tribe, but had to leave to be treated for polio. Harlan spent his childhood in hospitals and foster homes and now uses a motor scooter to get around. After retiring from the state penitentiary, he currently works as a substitute teacher and takes tickets at Cornhusker baseball games. Chamberlain still has family living in American Indian reservations.

According to Yankees television sportscaster Michael Kay, Joba's niece pronounced Justin as Joba and he grew to prefer this name. He eventually had his name legally changed to Joba.

Joba has a son, Karter, born in 2006.

Chamberlain is one of a few Native American baseball players currently in organized baseball; others include Bobby Madritsch, Kyle Lohse, and Jacoby Ellsbury.

The scouting reports on Chamberlain highlighted his power fastball (95-102), a plus-slider (86-90), a big, slow low-mid 70's curve which he rarely throws in the majors, and a developing change up. Joba has demonstrated good command of all but his change up, and has the potential to enter the elite starter ranks with two potentially plus-plus pitches.

However, the most exciting part about Joba's repertoire is not his velocity, but rather his mechanics and control. While he is able to break 100 mph at will, Joba has demonstrated the ability to paint the corners of the plate with a 97-99-mph fastball without appearing to overthrow the ball or break down his mechanics. Scouts rave more over his 90-mph slider, which has incredibly late bite and which he has superb control over, than his triple-digit fastball.

Chamberlain has quickly risen through the Yankee minor league organization and broke into the major leagues on August 7th, 2007. He struck out two batters and walked two in 2.0 IP, mainly using his fastball and slider. His fastball consistently registered from 95-98 and topped at 101 on the TV gun, while his slider consistently registered at 87 mph with a very sharp, late motion. His current strike out to innings pitched ratio implies that half of his outs are strikeouts.

According to Yankees announcer Micheal Kay, Joba wears the number 62 in honor of his best friend and childhood teammate who wore the number 8 and died from brain cancer. Throughout his minor league career he has said that his number has always added to eight (his friend's number) even when he hasn't asked for it.